# Chapter 7

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A 2-kg ball is moving with a speed of 4 m/s, and a 4-kg ball is moving with a speed of 2 m/s. What can you conclude about the kinetic energies of the two balls?
The 2-kg ball has more kinetic energy.
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A ball is dropped from a distance 5 m above the ground, and it hits the ground with a certain speed. If the same ball is dropped from a distance 10 m above ground, its final speed will be ________.
1.4 times as fast
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An object is dropped from a distance of R=2REarth from Earth's center, and it falls down and hits Earth's surface. If the object is dropped from a distance R=3REarth (twice as far away from Earth's surface), its kinetic energy right before hitting the ground would be _____ what it was before.
greater than the original value but less than twice
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If an object had a temperature of absolute zero (0 K), its ______________.
thermal energy would be zero
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A star forms when gravity causes a gas cloud to collapse. As the size of the cloud decreases, the temperature of the cloud _____________.
increases
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A positron is a particle similar to an electron, but with the opposite charge of an electron. If a positron and an electron collide, sometimes both the electron and positron disappear (annihilate), and two photons are created. This is an example of ____________.
the conversion of rest-mass energy into radiative energy
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A gas containing both hydrogen molecules (each molecule contains two hydrogen atoms) and helium atoms has a temperature of 300 K. How does the average speed of the hydrogen molecules compare to the helium atoms?
The average speed of the hydrogen molecules is faster than that of the helium atoms, but it is less than two times faster.
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What is the unit of work?
joule
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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?
Work
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How much work is done on a satellite in a circular orbit about Earth?
Zero
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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?
Both take the same 1000 J.
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Exactly what is it that enables an object to do work?
Energy
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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?
Lifted in the same time, the power is the same. The light sack moving in half the time requires double the power.
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The work that is done when twice the load is lifted twice the distance is _______.
four times as much.
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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?
It would have twice as much potential energy.
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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.
The car with twice the mass has twice the potential energy.
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When is the potential energy of something significant?
When the potential energy changes
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When one does twice the work in twice the time, the power expended is _______.
the same.
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When the speed of a moving car is doubled, how much more kinetic energy does it have?
It has four times as much.
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When you lift twice the load twice as high, in half the time, the increase in potential energy is _______.
four times.
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When traveling twice as fast your kinetic energy is increased _______.
by four.
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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?
It takes four times the work and four times the stopping distance.
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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?
300J
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How does speed affect the friction between a road and a skidding tire?
Speed has no effect on the friction.
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Energy cannot be _______.
destroyed
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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?
10 kJ
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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?
The energy is kinetic energy before it hits the ground; it is thermal energy after.
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What is the source of energy in sunshine?
Thermonuclear fusion energy
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What is recycled energy?
Reused energy that otherwise would be wasted
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In a simple machine, how much work is done when an input of 10 N acts over a distance of 5 m?
50 J
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Work
Force x Distance
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Work energy theorem
Wtotal = 1/2mvf^2 - 1/2mvi^2
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Net force
Work / distance
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Power
time it takes to do work, W/delta t
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units of power
j/s or watts
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unites of work
joules
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kilowatts
1 kw = 1000 watts = 1000 j/s
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GPE
work done by gravity, mgh
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EPE
spring energy, 1/2kx^2
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3 laws of work
you can't win, you can't break even, you are a loser
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the 6 simple machines
screw pulley lever ramp wheel & axel wedge
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IMA
factor by which your force gets reduced (ideal), distance effort/distance resistance
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MA