Week 4

24 July 2022
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question
A) Two identical cars are driving in opposite directions at the same speed. Their kinetic energies have _________. B) A motorcycle drives up a steeply inclined ramp. The work done on the motorcycle by Earth's gravitational force is _________. C) During a certain time interval, the net work done on an object is zero joules. We can be certain that _________. D) The amount of kinetic energy an object has depends on its mass and its speed. Rank the following sets of oranges and cantaloupes from least kinetic energy to greatest kinetic energy.
answer
A) the same magnitude and sign B) negative C) the object's final speed was the same as its initial speed D) mass: 4m speed: 1/4 v; total mass: 2 m speed: 1/2 v; mass: m speed: v; total mass: 4 m speed: v & mass: 4m speed: v
question
The work that is done when twice the load is lifted twice the distance is _________.
answer
four times as much
question
What is the unit of work?
answer
joule
question
A force sets an object in motion. When the force is multiplied by the time of its application, we call the quantity impulse, and an impulse changes the momentum of that object. What do we call the quantity force multiplied by distance?
answer
work
question
How much work is done on a satellite in a circular orbit about Earth?
answer
zero
question
Which requires more work: lifting a 50-kg sack a vertical distance of 2 m or lifting a 25-kg sack a vertical distance of 4 m?
answer
both take the same 1000 J
question
Exactly what is it that enables an object to do work?
answer
energy
question
If both sacks in the preceding question are lifted their respective distances in the same time, how does the power required for each compare? How about for the case in which the lighter sack is moved the same distance in half the time?
answer
lifted in the same time, the power is the same. the light sack moving in half the time requires double the power
question
When one does twice the work in twice the time, the power expended is _________.
answer
the same
question
A car is raised a certain distance in a service-station lift, thus giving it potential energy relative to the floor. If it were raised twice as high, how much more potential energy would it have?
answer
it would have twice as much potential energy
question
Two cars are raised to the same elevation on service-station lifts. If one car is twice as massive as the other, compare their gains of potential energy.
answer
the car with twice the mass has twice the gain of potential energy
question
When is the potential energy of something significant?
answer
when the potential energy changes
question
When you lift twice the load twice as high, in half the time, the increase in potential energy is _________.
answer
four times
question
When traveling twice as fast your kinetic energy is increased _________.
answer
by a factor of four
question
When the speed of a moving car is doubled, how much more kinetic energy does it have?
answer
it has four times as much
question
A) What happens the first time Dr. Hewitt lifts the bowling ball near his teeth and lets go? B) Why does the bowling ball behave the way it does the first time Dr. Hewitt lifts the bowling ball near his teeth and lets go? C) What happens the second time Dr. Hewitt lifts the bowling ball near his teeth and gives it a push? D) Why does the bowling ball behave as it does when Dr. Hewitt lifts it and gives it a push?
answer
A) the ball returns to Dr. Hewitt, stopping almost exactly at the point where it was released B) all of the initial energy of the ball was converted completely back to potential energy when the ball returned C) the ball leaves Dr. Hewitt and returns to him, going past the point where it was released D) the extra energy from the push is converted into kinetic energy, which is then converted into more potential energy at the end of the motion than the ball had when it was released
question
Energy cannot be _________.
answer
destroyed
question
Compared with a car moving at some original speed, how much work must the brakes of a car supply to stop a car that is moving twice as fast? How will the stopping distances compare?
answer
it takes four times the work and four times the stopping distance
question
If you push a crate horizontally with 100 N across a 10-m factory floor and the friction between the crate and the floor is a steady 70 N, how much kinetic energy does the crate gain?
answer
300 J
question
How does speed affect the friction between a road and a skidding tire?
answer
speed has no effect on the friction
question
Select the correct equations that show that 9.6 J of work is done when a force of 8.0 N moves a book 1.2 m.
answer
W=F*d=(8.0 N) * (1.2 m) = 9.6 J
question
Calculate the work done when a 26-N force pushes a cart 4.0 m.
answer
W=100 J
question
A) Calculate the work done in lifting a 460-N barbell 1.6 m above the floor. B) What is the gain of the potential energy of the barbell when it is lifted to this height?
answer
A) W= 740 J B) ΔU = 740 J
question
Select the correct equation that shows that 25 W of power is required to give a brick 100 J of PE in a time of 4 s.
answer
P=W/t=(100 J)/(4 s)= 25 W
question
Select the correct equations that show that when a 3.0-kg book is lifted 1.2 m its increase in gravitational potential energy is 36 J.
answer
PE=mgh=(3.0 kg) * (10 m/s^2) * (1/2 m) = 36 J
question
Select the correct equations that show the gravitational potential energy of a 1000-kg boulder raised 3 m above ground level is 30,000 J.
answer
PE=mgh=(1000 kg) * (10 N/kg) * (3 m) = 30,000 J
question
Calculate the kinetic energy of an 86-kg scooter moving at 12 m/s.
answer
KE=6200 J
question
Select the correct equations that show that 64 J of work is done when a 8.0-kg block of ice is moved from rest to a speed of 4.0 m/s.
answer
W=mv^2/2=((8.0 kg) * (4.0 m/s^2)/2= 64 J
question
No work is done by gravity on a bowling ball that is resting or moving on a bowling alley because the force of gravity on the ball acts perpendicular to the surface. But on an incline, the force of gravity has a vector component parallel to the alley, as in B. A) How does this component account for the acceleration of the ball? B) How does this component account for the work done on the ball to change its kinetic energy?
answer
A) in accord with Newton's second law, the component of gravitational force that is parallel to the incline in B produces an acceleration parallel to the incline B) in accord with the work-energy theorem, that parallel force component multiplied by the distance the ball travels is equal to the change in the ball's kinetic energy
question
The second floor of a house is 6 m above the street level. How much work is required to lift a 300-kg refrigerator to the second-story level?
answer
W=1.8x10^4 J
question
A) How much work is done when you push a crate horizontally with 150 N across a 15-m factory floor? B) If the force of friction on the crate is a steady 70 N, find the KE gained by the crate. C) Find the energy turned into heat.
answer
A) W=2300 J B) K=1200 J C) Q=1050 J
question
The work done in pushing a TV set a distance of 2 m with an average force of 20 N is _________.
answer
40 J
question
The work you do when pushing a shopping cart twice as far while applying the same force is _________.
answer
twice as much
question
No work is done by gravity on a bowling ball that rolls along a bowling alley because _________.
answer
the force on the ball is at right angles to the ball's motion
question
A crate of grapes lifted 10 meters gains 200 J of potential energy. If the same crate is instead lifted 20 meters, its gain in potential energy is _________.
answer
twice as much
question
A 2-kg box of taffy candy has 40 J of potential energy relative to the ground. Its height above the ground is _________.
answer
2 m
question
Two identical golf carts move at different speeds. The faster cart has twice the speed and therefore has _________.
answer
four times the kinetic energy
question
A melon is tossed straight upward with 100 J of kinetic energy. If air resistance is negligible the melon will return to its initial level with a kinetic energy of _________.
answer
100 J
question
When Joshua brakes his speeding bicycle to a stop, kinetic energy is transformed to _________.
answer
heat
question
A motorcycle moving at 50 km/h skids 10 m with locked brakes. How far will it skid with locked brakes when traveling at 150 km/h?
answer
90 m
question
About 40 J is required to push a crate 4 m across a floor. If the push is in the same direction as the motion of the crate, the force on the crate is about _________.
answer
10 N
question
Consider a bob attached by a string, a simple pendulum, that swings to and fro. A) Why doesn't the tension force in the string do work on the pendulum? B) Explain, however, why the force due to gravity on the pendulum at nearly every point does do work on the pendulum. C) What is the single position of the pendulum where "no work by gravity" occurs?
answer
A) tension in the string is everywhere perpendicular to the arc of the pendulum , with no component of tension B) in the case of gravity, a component of gravitational force on the pendulum exists parallel to the arc, which does work and changes the kinetic energy of the pendulum C) at the pendulum's lowest point
question
Consider a satellite in a circular orbit above Earth's surface. The force of gravity changes only the direction of motion of a satellite in circular motion (and keeps it in a circle); it does NOT change the satellite's speed. Work done on the satellite by the gravitational force is zero. What is your explanation?
answer
in circular orbit, the force of gravity is everywhere perpendicular to the satellite's motion. with no component of force parallel to its motion, no work is done and its kinetic energy remains constant
question
A) Which word in the statement of this problem allows you to assume that the table is frictionless? B) Suppose the potential energy of the block at the table is given by mgh/3. This implies that the chosen zero level of potential energy is _________. C) If the zero level is a distance 2h/3 above the floor, what is the potential energy U of the block on the floor? D) Considering that the potential energy of the block at the table is mgh/3 and that on the floor is -2mgh/3, what is the change in potential energy ΔU of the block if it is moved from the table to the floor? E) Which most simplified form of the law of conservation of energy describes the motion of the block when it slides from the top of the table to the bottom of the ramp? F) As the block slides down the ramp, what happens to its kinetic energy K, potential energy U, and total mechanical energy E? G) Using conservation of energy, find the speed vb of the block at the bottom of the ramp. H) Which most simplified form of the law of conservation of energy describes the motion of the block as it slides on the floor from the bottom of the ramp to the moment it stops? I) As the block slides across the floor, what happens to its kinetic energy K, potential energy U, and total mechanical energy E? J) What force is responsible for the decrease in the mechanical energy of the block? K) Find the amount of energy E dissipated by friction by the time the block stops.
answer
A) smooth B) a distance 2h/3 above the floor C) U= -2mgh/3 D) ΔU = -mgh E) 1/2mv2i+mghi=1/2mv2f+mg F) K increases; U decreases; E stays the same G) vb = √(v^2)+2gh H) 1/2mv2/i+Wnc=0 I) K decreases; U stays the same; E decreases J) friction K) E= 1/2mv^2+mgh
question
A) Where on the track is the skater's kinetic energy the greatest? B) As the skater is skating back and forth, where does the skater have the most potential energy? C) Observe the total energy bar on the Bar Graph. As the skater is skating back and forth, which statement best describes the total energy? D) Match the approximate numerical values on the left with the energy type categories on the right to complete the equations. Assume that the mass of the skater is 75 kg and that the acceleration of gravity is 9.8 N/kg. E) Based on the previous question, which statement is true? F) If the skater started from rest 4 m above the ground (instead of 7m), what would be the kinetic energy at the bottom of the ramp (which is still 1 m above the ground)? G) Consider again the case where the skater starts 7 m above the ground and skates down the track. What is the skater's speed when the skater is at the bottom of the track? H) When the skater starts 7 m above the ground, how does the speed of the skater at the bottom of the track compare to the speed of the skater at the bottom when the skater starts 4 m above the ground? I) If the skater starts from rest at position 1, rank, in increasing order from least to greatest, the kinetic energy of the skater at the five positions shown.
answer
A) the skater's kinetic energy is ... at its maximum value at the lowest point of the track B) the skater's potential energy is ... at its maximum value at the locations where the skater turns and goes back in the opposite direction C) the total energy is ... the same at all locations of the track D) total energy at initial position= 5145 J potential energy at initial position= 5145 J kinetic energy at initial position = 0 J total energy at bottom of track = 5145 J potential energy at bottom of track= 735 J kinetic energy at bottom of track = 4410 J E) the kinetic energy at the bottom of the ramp is ... equal to the amount of potential energy loss in going from the initial location to the bottom F) 2205 J G) 11 m/s H) the speed is ... higher, but less than twice as fast I) 1, 3, 5&2, 4
question
Suppose our experimenter repeats his experiment on a planet more massive than Earth, where the acceleration due to gravity is g= 30 m/s^2. When he releases the ball from chin height without giving it a push, how will the ball's behavior differ from its behavior on Earth? Ignore friction and air resistance.
answer
it will take less time to return to the point from which it was released
question
In a simple machine, how much work is done when an input of 10 N acts over a distance of 5 m?
answer
50 J
question
What will the kinetic energy of a pile driver ram be if it starts from rest and undergoes a 10 kJ decrease in potential energy?
answer
10 kJ
question
An apple hanging from a limb has potential energy because of its height. If it falls, what becomes of this energy just before it hits the ground? When it hits the ground?
answer
immediately before hitting the ground the apple's energy is kinetic energy; when it hits the ground, its energy becomes thermal energy
question
What is the source of energy in sunshine?
answer
thermonuclear fusion energy
question
What is recycled energy?
answer
reused energy that otherwise would be wasted
question
When the useful energy output of a simple machine is 100 J, and the total energy input is 200 J, the efficiency is _________.
answer
50%
question
Can a machine multiply input force? Input distance? Input energy?
answer
force: yes distance: yes energy: no
question
If a machine multiplies force by a factor of 4, what other quantity is diminished, and by how much?
answer
distance is diminished to one-quarter
question
A force of 50 N is applies to the end of a lever, which is moved a certain distance. If the other end of the lever moves one-third as far, how much force does it exert?
answer
150 N
question
Recycled energy is mainly _________.
answer
use of energy otherwise wasted
question
What is the efficiency of a machine that miraculously converts all the input energy to useful output energy?
answer
100%
question
If an input of 100 J in a pulley system increases the potential energy of a load by 60 J, what is the efficiency of the system?
answer
60%
question
Which energy production method does not ultimately depend on the Sun?
answer
nuclear fission
question
Fossil fuels, hydroelectric power, and wind power ultimately get their energy from _________.
answer
the Sun
question
What is the ultimate source of the energy from fossil fuels, dams, and windmills?
answer
the Sun
question
What is the ultimate source of geothermal energy?
answer
nuclear power in Earth's interior
question
Can we correctly say that hydrogen is a new source of energy? Why or why not?
answer
no, it takes energy to make hydrogen
question
Select the correct equations that show that a machine that has an input of 100 J and an output of 25 J is 25% efficient.
answer
Efficiency η=(Eout/Ein)⋅100%=((25J)/(100J))⋅100%=25%
question
The roller coaster ride starts from rest at point A. A) Rank speed from greatest to least at each point. B) Rank KE from greatest to least at each point. C) Rank PE form greatest to least at each point.
answer
A) d, b, c, e, a B) d, b, c, e, a C) a, e, c, b, d
question
Belly-flop Bernie dives from atop a a tall flagpole into a swimming pool below. His potential energy at the top is 12,000 J (relative to the surface of the pool). What is his kinetic energy when his potential energy reduces to 1,000 J?
answer
KE= 1.1x10^4 J
question
Nellie Newton applies a force of 58 N to the end of a lever, which is moved a certain distance. If the other end of the lever moves one-third as far, find the force it exerts.
answer
F= 170 N
question
Consider an ideal pulley system. If you pull one end of the rope 0.60 m downward with a 46-N force, find the height you can lift a 230-N load.
answer
h= 0.12 m
question
In raising a 4,200-N piano with a pulley system, the workers note that for every 2.1 m of rope pulled downward, the piano rises 0.22 m. Ideally, find the force that is required to lift the piano.
answer
F= 440 N
question
In the hydraulic machine shown, you observe that when the small piston is pushed down 10 cm, the large piston is raised 1 cm. If the small piston is pushed down with a force of 100 N, what is the most weight that the large piston can support?
answer
W= 1,000 N
question
Emily holds a banana of mass m over the edge of a bridge of height h. She drops the banana and it falls to the river below. Use conservation of energy to find the speed of the banana just before hitting the water.
answer
v= √2gh
question
A ball rolling down an incline has its maximum potential energy at _________.
answer
the top
question
A ball rolling down an incline has its maximum kinetic energy at _________.
answer
the bottom
question
The bob of a simple pendulum has its maximum kinetic energy at the _________.
answer
bottom of its swing
question
After rolling halfway down an incline, a marble's kinetic energy is _________.
answer
the same as its potential energy
question
A block of ice sliding down an incline has half its maximum kinetic energy _________.
answer
halfway down
question
A light aluminum ball and a heavy lead ball of the same size roll down an incline. When they are halfway down the incline, they will have identical _________.
answer
(momentum, potential energies, inertias, kinetic energies) none of the above
question
A circus diver drops from a high pole into water far below. When he is halfway down _________.
answer
(he has gained an amount of kinetic energy equal to half his initial potential energy, his potential energy is halved, his kinetic energy and potential energy are equal) all of the above
question
Acrobat Bart at the circus drops vertically onto the end of a see-saw, with his partner Art equidistant from the fulcrum at the other end. Art is propelled straight upward a distance twice that of Bart's dropping distance. Neglecting inefficiencies we see _________.
answer
Art has half the mass of Bart
question
A hydraulic jack is used to lift objects such as automobiles. If the input force is 200 N over a distance of 1 meter, the output force over a distance of 0.1 meter is ideally _________.
answer
2,000 N
question
A hydraulic press has its input piston depressed 20 centimeters while the output piston is raised 1 centimeter. A 1-newton input can lift a load of _________.
answer
20 N
question
A machine puts out 100 watts of power for every 1,000 watts put into it. The efficiency of the machine is _________.
answer
10%
question
A jack system will increase the potential energy of a heavy load by 1,000 J with a work input of 2,000 J. The efficiency of the jack system is _________.
answer
50%
question
The tangential speed on the outer edge of a rotating carousel is _________.
answer
greater than toward the center
question
What are the units of measurement for tangential speed? For rotational speed?
answer
m/s for tangential, RPM for rotational
question
On a rotating turntable, how do tangential speed and rotational speed vary with distance from the center?
answer
tangential speed increases with distance; rotational speed is constant
question
A tapered cup rolled on a flat surface makes a circular path. What does this tell you about the tangential speed of the rim of the wide end of the cup compared with that of the rim of the narrow end?
answer
the tangential speed of the wide end is faster
question
How does the tapered rim of a wheel on a railroad allow one part of the rim to have a greater tangential speed than another part when it is rolling on a track?
answer
the tangential speed is proportional to the radius the inside of the wheel rolls on a larger radius than the outside of the wheel
question
Rotational inertia about the midpoint of an object becomes greater with _________.
answer
increased mass and increased distance to mass concentration
question
How is a flywheel constructed to maximize its rotational inertia?
answer
most of the mass is concentrated far from the axis
question
Inertia depends on mass; rotational inertia depends on mass and something else. What?
answer
the distribution of mass about the axis of rotation
question
As distance increases between most of the mass of an object and its center of rotation, how does rotational inertia change?
answer
it increases
question
Consider three axes of rotation for a pencil: along the lead, at right angles to the lead at the middle, and at right angles to the lead at one end. Rate the rotational inertias about each axis from smallest to largest.
answer
axis along the head, through the center, through one end
question
Which is easier to get swinging: a baseball bat held at the narrow end or a bat held closer to the massive end (choked up)? Also answer for a short bat versus a long bat.
answer
the easiest is choked up and a short bat
question
Why does bending your legs when running enable you to swing your legs to and fro more rapidly?
answer
bending your legs shortens them, thus reducing rotational inertia
question
Which will have the greater acceleration rolling down an incline: a hoop or a solid disk? Why?
answer
the solid disk will because the mass is closer to the axis of rotation
question
A) Is it easier to balance a long rod with a mass attached to it when the mass is closer to your hand or when the mass is further away? B) How does the rotational inertia of the rod with the mass toward the bottom compare with the rotational inertia of the mass toward the top? C) Why does the rotational inertia of the rod with the attached mass closer to your hand compare the way it does with the rotational inertia of the the rod with the attached mass further away? D) Is it easier for a circus performer to balance a long rod held vertically with people hanging off the other end, or the same long rod without the people at the other end, and why?
answer
A) it is easier when the mass is farther from your hand B) the rotational inertia of the rod with the mass closer to the top is greater than the rotational inertia of the rod with the mass closer to the bottom C) rotational inertia depends on whether the mass is farther or closer to the point of rotation. the farther the mass is, the higher the rotational inertia D) it is easier for the performer to balance a long rod held vertically with people at the other end because the rotational inertia is greater
question
A) How does the lever arm change if you decrease the angle of the force? B) How does the lever arm change when you decrease the distance to the nut? C) How does the force needed to turn the wrench change if you increases the lever arm?
answer
A) it decreases B) it decreases C) it decreases
question
The figure below shows two blocks suspended by a single cord over a pulley. The mass of block B is twice the mass of block A, while the mass of the pulley is equal to the mass of block A. The blocks are let free to move and the cord moves on the pulley without slipping or stretching. There is no friction in the pulley axle, and the cord's weight can be ignored. A) Which of the following statements correctly described the system shown in the figure? B) What happens when block B moves downward?
answer
A) the angular acceleration of the pulley is nonzero B) the cord's tension on the right side of the pulley is higher than on the left side
question
The torque exerted by a crowbar on an object increases with increased _________.
answer
force and leverage distance
question
What does a torque tend to do to an object?
answer
torque tends to twist or change the state of rotation of the object
question
What is meant by the "lever arm" of a torque?
answer
it is the perpendicular distance from the rotational axis to the line along which the force acts
question
How do clockwise and counterclockwise torques compare when a system is balanced?
answer
they are equal in magnitude and opposite in direction
question
Suppose we replace the mass in the video with one that is four times heavier. How far from the free end must we place the pivot to keep the meter stick in balance?
answer
90 cm (10 cm from the weight)
question
A) Where is the center of gravity of the broom that Dr. Hewitt holds up? B) When Dr. Hewitt cuts the broom right through the center of gravity, how do the weights of the two sides of the broom compare? C) How does the torque due to the weight of one side of the broom exerted around the balance point compare with the torque exerted by the weight of the other side of the broom around the balance point?
answer
A) at the balance point B) the shorter side, where the bristles of the broom are, has a greater weight than the handle C) the torque due to the weight of the shorter side (the bristles of the broom) is equal in magnitude to the torque due to the weight of the longer side, and opposite in direction
question
The center of gravity of a basketball is located _________.
answer
at its geometrical center
question
If you toss a stick into the air, it appears to wobble all over the place. Specifically, about what place does it wobble?
answer
it will rotate about the center of mass
question
Where is the center of mass of a baseball? Where is its center of gravity?
answer
the center of mass and the center of gravity are both at the center of the spherical ball
question
If you hang at rest by your hands from a long vertical rope, where is your center of gravity with respect to the rope?
answer
somewhere straight below where your hands grab the rope
question
Where is the center of mass of a hollow soccer ball?
answer
in the center of the ball
question
What is the relationship between the center of gravity and the support base for an object that is in stable equilibrium?
answer
a vertical line through the center of gravity must pass inside the support base of the object
question
Why doesn't the Leaning Tower of Pisa topple over?
answer
a vertical line through the center of gravity passes inside its support base
question
Why can't you stand with your heels and back to a wall and then bend over to touch your toes and return to your stand-up position?
answer
a vertical line through the center of gravity of your body is in front of the tips of your toes, which is outside your support base
question
In the video, the torque due to the mass of the plank is used in the calculations. For this question, ignore the mass of the board. Rank, from largest to smallest, the mass m needed to keep the board from tipping over. A: 1.5 m, 1.5 m, 100 kg B: 0.75 m, 0.75 m, 100 kg C: 1.5 m, 0.75 m, 100 kg D: 1.5 m, 1.5 m, 200 kg E: 1.5 m, 0.5 m, 100 kg
answer
d, a&b, c, e
question
Calculate the torque produced by a 60-N force when a pipe extends the length of the wrench to 0.50 m.
answer
T= 30 N*m
question
A) Two racecars are driving at constant speeds around a circular track. Both cars are the same distance away from the center of the track, but car 2 is driving twice as fast as car 1. The acceleration of car 2 is _________ the acceleration of car 1. B) Two children are riding on a merry-go-round that is rotating with a constant angular speed. Abbie is one meter from the center of the merry-go-round, whereas Zak is two meters from the center. Abbie's acceleration is _________ Zak's acceleration. C) Four racecars are driving at constant speeds around a circular racetrack. The data table gives the speed of each car and each car's distance from the center of the track at one instant. a: 40, 20 b: 40, 25 c: 50, 20 d: 50, 25 D) Cheryl is riding on the edge of a merry-go-round, 2 m from the center, which is rotating with an increasing angular speed. Cheryl's tangential acceleration is 3.0 m/s^2. At the instant that Cheryl's linear speed is 4.0 m/s, what is Cheryl's total acceleration?
answer
A) four times B) half of C) acceleration of car C, acceleration of car D, acceleration of car A, acceleration of car B D) 8.5 m/s^2
question
A) A baseball bat can be rotated around many different axes of rotation. Three such possibilities are shown in Figure 1. Rank the baseball bat's moment of inertia about each of these three axes of rotation. a: center of bat b: end of handle c: length of whole bat B) Given the same baseball bat and possible axes of rotation shown in Figure 1, for which axis of rotation would it be the easiest to rotate the bat from rest? C) A girl spins around in a circle trying to make herself dizzy. Without changing her position, she starts spinning twice as fast. By how much did her rotational kinetic energy change? D) A solid sphere and a hollow sphere (spherical shell) of the same mass and same radius rotate with the same amount of kinetic energy. Which one is rotating faster?
answer
A) b, a, c B) c C) it quadrupled D) the solid sphere
question
The two wheels in Figure 1 are touching; turning the larger one causes the smaller one to turn as well without sliding. A) If the larger wheel turns clockwise, is the angular velocity of the smaller wheel positive or negative using the convention described in the video? B) The contact between the two wheels means that the following quantities are related. Sort these quantities by whether they are the same or different for the two wheels. C) If the larger wheel rotates with angular speed ωB, what can you say about the angular speed of the smaller wheel? D) Which turbine takes the lesser amount of time to rotate through 1.0 radian of angular displacement? E) As in Part D, two wind turbines with different length blades are rotating. Consider what needs to happen in order to change the angular speed of one of the turbines. If the turbine is to spin more quickly, should the angular acceleration, α, be positive or negative?
answer
A) positive B) same for both wheels: tangential speed at the outer edge of the large wheel & tangential speed at the outer edge of the small wheel different for each wheel: time to complete one revolution & angular speed C) the angular speed of the smaller wheel is greater than ωB. D) they take the same amount of time E) we cannot tell which direction α should be without knowing the direction of the angular velocity ω
question
Dan and Sue cycle at the same speed. The tires on Dan's bike are larger in diameter than those on Sue's bike. Which wheels, if either, have the greater rotational speed?
answer
Sue's wheels have the greater rotational speed
question
Can a force produce a torque when there is no lever arm?
answer
no
question
The centers of gravity of the three trucks parked on a hill are shown by the Xs. Which truck will tip over?
answer
the left truck
question
You hold a meterstick horizontally with the same mass suspended at the end. Rank the torque needed to keep the stick steady, from largest to smallest. a: pointed upward b: straight out c: pointed downward
answer
b, c, a
question
The diameter of the base of a tapered drinking cup is 6.8 cm. The diameter of its mouth is 8.6 cm. The path of the cup curves when you roll it on the top of a table. A) Which end, the base or the mouth, rolls faster? B) How much faster?
answer
A) the mouth B) vm/vb= 1.3
question
To tighten a bolt, you push with a force of 76 N at the end of a wrench handle that is 0.28 m from the axis of the bolt. A) What torque are you exerting? B) If you move your hand inward to be only 0.10 m from the bolt, find the force that you should exert to achieve the same torque. C) Do your answers depend on the direction of your push relative to the direction of the wrench handle?
answer
A) T= 21 N*m B) F= 210 N C) yes
question
The rock and meterstick balance at the 25-cm mark, as shown. The meterstick has a mass of 1.3 kg. What must be the mass of the rock?
answer
m= 1.3 kg
question
When a rock tied to a string is whirled in a horizontal circle, doubling the speed _________.
answer
quadruples the tension in the string
question
When you whirl a can at the end of a string in a circular path, what is the direction of the force you exert on the can?
answer
toward the center of the circle
question
Is it an inward force or an outward force that is exerted on the clothes during the spin cycle of an automatic washing machine? What about the direction of the force on the water?
answer
the force is inward on clothes and there is none on water
question
When a rock tied to a string whirled in a horizontal circle somehow doubles in mass but keeps the same speed, the string tension _________.
answer
doubles
question
If the string that holds a whirling can in its circular path breaks, what kind of force causes it to move in a straight-line path: centripetal, centrifugal, or no force? What law of physics supports your answer?
answer
no force; Newton's first law
question
If you are not wearing a seat belt in a car that rounds a curve, and you slide across your seat and slam against a car door, what kind of force is responsible for your slide: centripetal, centrifugal, or no force?
answer
there is no force as viewed by someone outside the car to them you move in a straight line
question
Why is centrifugal force in a rotating frame called a "fictitious force"?
answer
it is not a fundamental force of nature instead, it is a force that only appears in an accelerating frame of reference
question
How can gravity be simulated in an orbiting space station?
answer
(spin a station shaped like a bicycle wheel, spin two pods connected by a cable, rotate a cylinder to create centrifugal force as viewed by a person on the inside of the curved outer wall) all of the above
question
Consider someone in a rotating space habitat. The outward force felt by the person _________.
answer
has no reaction counterpart
question
Distinguish between linear momentum and angular momentum.
answer
angular momentum depends on the distribution of mass, whereas linear momentum depends on the total mass
question
What is the law of inertia for rotating systems in terms of angular momentum?
answer
angular momentum does not change without an applied external torque
question
Consider someone in a rotating space habitat. Rotational motion provides _________.
answer
a support force
question
What is required to change the angular momentum of a system?
answer
external torque
question
When the rotational speed of a rotating system doubles, its angular momentum _________.
answer
doubles
question
If a skater who is spinning pulls her arms in so as to reduce her rotational inertia by half, how much will her angular momentum change? By how much will her rate of spin change?
answer
her angular momentum does not change her spin rate doubles
question
The experimenter from the video rotates on his stool, this time holding his empty hands in his lap. You stand on a desk above him and drop a long, heavy bean bag straight down so that it lands across his lap, in his hands. What happens?
answer
he spins slower
question
Calculate the torque produced by a 48-N perpendicular force at the end of a 0.19-m long wrench. torque=lever arm x force
answer
T= 9.1 Nm
question
Calculate the tension in a horizontal string that whirls a 2.2-kg toy in a circle of radius 2.0 m when it moves at 2.6 m/s on an icy surface. centripetal force: F= mv^2/r
answer
F= 7.4 N
question
If a trapeze artist rotates once each second while sailing through the air and contracts to reduce her rotational inertia to one-third of what it was, how many rotations per second will result?
answer
ω= 3.00 rotations per second
question
A small space telescope at the end of a tether line of length L moves at linear speed v about a central space station. A) What will be the linear speed of the telescope if the length of the line is reduced to 0.33 L? B) If the initial linear speed of the telescope is 0.70 m/s, what is its speed when pulled in to one-third its initial distance from the space station?
answer
A) vnew= v/0.33 B) 2.1 m/s
question
As you crawl toward the edge of a large freely-rotating horizontal turntable in a carnival funhouse, the angular momentum of you and the turntable _________.
answer
remains the same, but the revolutions per minute decrease
question
As a huge rotating cloud of particles in space gravitate together forming an increasingly dense ball, it shrinks in size and _________.
answer
rotates faster
question
When you do somersaults, you'll more easily rotate when your body is _________.
answer
curled into a ball shape
question
The chef at the infamous Fattening Tower of Pizza tosses a spinning disk of uncooked pizza dough into the air. The disk becomes wider during its flight, while its rotational speed _________.
answer
slows
question
When a twirling ice skater brings her arms inward, her rotational speed _________.
answer
increases
question
If the rotational speed of a rotating habitat in space increases, the weight experienced by occupants _________.
answer
increases
question
Multiply the equation for linear momentum by radial distance r and you have _________.
answer
angular momentum
question
A difference between linear momentum and angular momentum involves _________.
answer
(a radial distance, two types of speed) both of these
question
The force responsible for a car moving steadily along a no-friction icy banked curve is due to _________.
answer
the radial component of the normal force
question
Centrifugal forces are an apparent reality to observers in a reference frame that is _________.
answer
rotating
question
Compared with a force, a torque involves _________.
answer
(rotation, leverage, distance from an axis of rotation) all the above
question
A torque acting on an object tends to produce _________.
answer
rotation
question
If Alex wishes to rotate his skateboard, then he must apply a _________.
answer
torque
question
If you place a pipe over the end of a wrench when trying to rotate a stubborn bolt, effectively making the wrench handle twice as long, you'll multiply the torque by _________.
answer
two
question
For a system in mechanical equilibrium, the resultant _________.
answer
(force must be zero, torque must be zero) both of these
question
To rotate a stubborn screw, it is best to use a screwdriver that has a _________.
answer
wide handle
question
Two people are balanced on a seesaw. If one person leans inward toward the center of the seesaw, that person's end of the seesaw tends to _________.
answer
rise
question
On a balanced seesaw, a boy three times as heavy as his partner sits _________.
answer
1/3 the distance from the fulcrum
question
A 1-kg rock is suspended from the tip of a horizontal meterstick at the 0-cm mark so that the meterstick barely balances like a seesaw when its fulcrum is at the 25-cm mark. From this information, the mass of the meterstick is _________.
answer
1 kg
question
If you balance a broom horizontally on one finger, the center of gravity of the broom will be above your finger, closer to the bristles end than the handle end. If you saw the broom in two pieces at that point and weigh the two pars on a scale, you'll find that the heavier part is the _________.
answer
bristles part