# Mastering Physics Chapter 7

## Unlock all answers in this set

question
The work that is done when twice the load is lifted twice the distance is
four times as much
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When one does twice the work in twice the time, the power expended is
the same
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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 gain of potential energy
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What happens the first time Dr. Hewitt lifts the bowling ball near his teeth and lets go?
The ball returns to Dr. Hewitt, stopping almost exactly at the point where it was released.
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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?
All of the initial energy of the ball was converted completely back to potential energy when the ball returned.
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What happens the second time Dr. Hewitt lifts the bowling ball near his teeth and gives it a push?
The ball leaves Dr. Hewitt and returns to him, going past the point where it was released.
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Why does the bowling ball behave as it does when Dr. Hewitt lifts it and gives it a push?
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.
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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 a factor of four.
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Energy cannot be _______.
destroyed.
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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?
300 J
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Which word in the statement of this problem allows you to assume that the table is frictionless?
smooth
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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 __________.
a distance 2h/3 above the floor
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If the zero level is a distance 2h/3 above the floor, what is the potential energy U of the block on the floor?
U = −2mgh/3
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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?
ΔU = −mgh
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Which 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?
12mv2i+mghi=12mv2f+mghf
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As the block slides down the ramp, what happens to its kinetic energy K, potential energy U, and total mechanical energy E?
K increases;U decreases;E stays the same
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As the block slides across the floor, what happens to its kinetic energy K, potential energy U, and total mechanical energy E?
K decreases;U stays the same;E decreases
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What force is responsible for the decrease in the mechanical energy of the block?
friction
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Suppose our experimenter repeats his experiment on a planet more massive than Earth, where the acceleration due to gravity is g=30 m/s2. 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. (Select all that apply.)
It will take less time to return to the point from which it was released.
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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?
10KJ
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When the useful energy output of a simple machine is 100 J, and the total energy input is 200 J, the efficiency is _______.
50%
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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?
60%
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Fossil fuels, hydroelectric power, and wind power ultimately get their energy from _______.
the Sun
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Select the correct equations that show that 7.2 J of work is done when a force of 6.0 N moves a book 1.2 m.
W=F⋅d=(6.0N)⋅(1.2m)=7.2J
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Calculate the kinetic energy of an 88-kg scooter moving at 12 m/s .
KE= 6300 J
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If a golf ball and a Ping-Pong ball both move with the same kinetic energy, can you say which has the greater speed? Explain in terms of the definition of KE. Similarly, in a gaseous mixture of heavy molecules and light molecules with the same average KE, can you say which have the greater speed?
If KEs are the same but masses differ, then the ball with smaller mass has the greater speed. That is, 1⁄2Mv^2 = 1⁄2mV^2. Likewise with molecules, where lighter ones move faster on the average than more massive ones.
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Why is it easier to stop a lightly loaded truck than a heavier one that has equal speed?
It has less kinetic energy so it needs less work to stop
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What is the unit of work?
joule
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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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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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Can a machine multiply input force? Input distance? Input energy?
Force and distance, but not energy
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A force of 50 N is applied 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?