Week 4

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.

The work that is done when twice the load is lifted twice the distance is _________.

What is the unit of work?

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?

How much work is done on a satellite in a circular orbit about Earth?

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?

Exactly what is it that enables an object to do work?

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?

When one does twice the work in twice the time, the power expended is _________.

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?

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.

When is the potential energy of something significant?

When you lift twice the load twice as high, in half the time, the increase in potential energy is _________.

When traveling twice as fast your kinetic energy is increased _________.

When the speed of a moving car is doubled, how much more kinetic energy does it have?

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?

Energy cannot be _________.

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?

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?

How does speed affect the friction between a road and a skidding tire?

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.

Calculate the work done when a 26-N force pushes a cart 4.0 m.

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?

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.

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.

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.

Calculate the kinetic energy of an 86-kg scooter moving at 12 m/s.

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.

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?

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?

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.

The work done in pushing a TV set a distance of 2 m with an average force of 20 N is _________.

The work you do when pushing a shopping cart twice as far while applying the same force is _________.

No work is done by gravity on a bowling ball that rolls along a bowling alley because _________.

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 _________.

A 2-kg box of taffy candy has 40 J of potential energy relative to the ground. Its height above the ground is _________.

Two identical golf carts move at different speeds. The faster cart has twice the speed and therefore has _________.

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 _________.

When Joshua brakes his speeding bicycle to a stop, kinetic energy is transformed to _________.

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?

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 _________.

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?

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?

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.

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.

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.

In a simple machine, how much work is done when an input of 10 N acts over a distance of 5 m?

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?

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?

What is the source of energy in sunshine?

What is recycled energy?

When the useful energy output of a simple machine is 100 J, and the total energy input is 200 J, the efficiency is _________.

Can a machine multiply input force? Input distance? Input energy?

If a machine multiplies force by a factor of 4, what other quantity is diminished, and by how much?

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?

Recycled energy is mainly _________.

What is the efficiency of a machine that miraculously converts all the input energy to useful output energy?

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?

Which energy production method does not ultimately depend on the Sun?

Fossil fuels, hydroelectric power, and wind power ultimately get their energy from _________.

What is the ultimate source of the energy from fossil fuels, dams, and windmills?

What is the ultimate source of geothermal energy?

Can we correctly say that hydrogen is a new source of energy? Why or why not?

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.

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.

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?

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.

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.

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.

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?

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.

A ball rolling down an incline has its maximum potential energy at _________.

A ball rolling down an incline has its maximum kinetic energy at _________.

The bob of a simple pendulum has its maximum kinetic energy at the _________.

After rolling halfway down an incline, a marble's kinetic energy is _________.

A block of ice sliding down an incline has half its maximum kinetic energy _________.

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 _________.

A circus diver drops from a high pole into water far below. When he is halfway down _________.

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 _________.

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 _________.

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 _________.

A machine puts out 100 watts of power for every 1,000 watts put into it. The efficiency of the machine is _________.

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 _________.

The tangential speed on the outer edge of a rotating carousel is _________.

What are the units of measurement for tangential speed? For rotational speed?

On a rotating turntable, how do tangential speed and rotational speed vary with distance from the center?

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?

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?

Rotational inertia about the midpoint of an object becomes greater with _________.

How is a flywheel constructed to maximize its rotational inertia?

Inertia depends on mass; rotational inertia depends on mass and something else. What?

As distance increases between most of the mass of an object and its center of rotation, how does rotational inertia change?

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.

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.

Why does bending your legs when running enable you to swing your legs to and fro more rapidly?

Which will have the greater acceleration rolling down an incline: a hoop or a solid disk? Why?

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?

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?

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?

The torque exerted by a crowbar on an object increases with increased _________.

What does a torque tend to do to an object?

What is meant by the "lever arm" of a torque?

How do clockwise and counterclockwise torques compare when a system is balanced?

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?

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?

The center of gravity of a basketball is located _________.

If you toss a stick into the air, it appears to wobble all over the place. Specifically, about what place does it wobble?

Where is the center of mass of a baseball? Where is its center of gravity?

If you hang at rest by your hands from a long vertical rope, where is your center of gravity with respect to the rope?

Where is the center of mass of a hollow soccer ball?

What is the relationship between the center of gravity and the support base for an object that is in stable equilibrium?

Why doesn't the Leaning Tower of Pisa topple over?

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?

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

Calculate the torque produced by a 60-N force when a pipe extends the length of the wrench to 0.50 m.

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?

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?

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?

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?

Can a force produce a torque when there is no lever arm?

The centers of gravity of the three trucks parked on a hill are shown by the Xs. Which truck will tip over?

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

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?

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?

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?

When a rock tied to a string is whirled in a horizontal circle, doubling the speed _________.

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?

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?

When a rock tied to a string whirled in a horizontal circle somehow doubles in mass but keeps the same speed, the string tension _________.

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?

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?

Why is centrifugal force in a rotating frame called a "fictitious force"?

How can gravity be simulated in an orbiting space station?

Consider someone in a rotating space habitat. The outward force felt by the person _________.

Distinguish between linear momentum and angular momentum.

What is the law of inertia for rotating systems in terms of angular momentum?

Consider someone in a rotating space habitat. Rotational motion provides _________.

What is required to change the angular momentum of a system?

When the rotational speed of a rotating system doubles, its angular momentum _________.

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?

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?

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

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

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?

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?

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 _________.

As a huge rotating cloud of particles in space gravitate together forming an increasingly dense ball, it shrinks in size and _________.

When you do somersaults, you'll more easily rotate when your body is _________.

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 _________.

When a twirling ice skater brings her arms inward, her rotational speed _________.

If the rotational speed of a rotating habitat in space increases, the weight experienced by occupants _________.

Multiply the equation for linear momentum by radial distance r and you have _________.

A difference between linear momentum and angular momentum involves _________.

The force responsible for a car moving steadily along a no-friction icy banked curve is due to _________.

Centrifugal forces are an apparent reality to observers in a reference frame that is _________.

Compared with a force, a torque involves _________.

A torque acting on an object tends to produce _________.

If Alex wishes to rotate his skateboard, then he must apply a _________.

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 _________.

For a system in mechanical equilibrium, the resultant _________.

To rotate a stubborn screw, it is best to use a screwdriver that has a _________.

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 _________.

On a balanced seesaw, a boy three times as heavy as his partner sits _________.

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 _________.

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 _________.