# Units 5 Review

Work, Energy and Power

 Units 5 Review - Questions Only Navigate to Answers for:  [ #17 | #18 | #19 | #20 | #21 | #22 | #23 | #24 | #25 | #26 | #27 | #28 | #29 | #30 | #31 ]

17. A moving object has __________.
 a. speed b. velocity c. momentum d. energy e. all of these

A moving object has speed and velocity. Since momentum is mass*velocity, a moving object would also has momentum. And since a moving object has kinetic energy (0.5*m*v^2), it would also have energy.

Momentum (7 seconds) | Kinetic Energy (4 seconds)

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18. An object at rest may have __________.
 a. speed b. velocity c. momentum d. energy e. all of these

An object at rest absolutely cannot have speed, velocity or momentum. However, an object at rest could have energy if there is energy stored due to its position; for example, there could be gravitational or elastic potential energy.

Momentum (7 seconds) | Mechanical Energy (15 seconds)

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19. What does an object have when it is moving that it absolutely doesn`t have when at rest?
 a. momentum b. energy c. mass d. inertia e. none of these

When moving, an object has momentum; it definitely would not have this if it were at rest. An object which is moving would have kinetic energy (one of the two forms of mechanical energy); but if at rest, it could still have energy of position (potential energy) even though it would not have energy of motion (kinetic energy). An object has mass and inertia whether it is moving or not.

Inertia (12 seconds) | Momentum (7 seconds) | Mechanical Energy (15 seconds)

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20. If an object has kinetic energy, then it also must have ___________.
 a. impulse b. momentum c. acceleration d. force e. none of these

An object which has kinetic energy has mass and speed (or velocity). For this reason, it would also have momentum.

Momentum (7 seconds) | Kinetic Energy (4 seconds)

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21. If the speed of a moving object doubles, then what else doubles?
 a. momentum b. kinetic energy c. acceleration d. all of the above e. none of these

Momentum is directly proportional to the speed of the object; so if the speed is doubled, the momentum is doubled. However, kinetic energy is directly proportional to the square of the speed; thus, doubling the speed would serve to quadruple the kinetic energy.

Acceleration (12 seconds) | Momentum (7 seconds) | Kinetic Energy (4 seconds)

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22. A popular swinging-balls apparatus consists of an aligned row of identical elastic balls that are suspended by strings so they barely touch each other. When two balls are lifted from one end and released, they strike the row and two balls pop out from the other end. If instead one ball popped out with twice the velocity of the two, this would be violation of conservation of __________.
 a. momentum b. kinetic energy c. both of these d. none of these

Momentum would be conserved in such an instance; yet, kinetic energy would not be conserved in the collision. Consider the balls to have a mass of m (for each ball) and an initial velocity of v. The initial momentum and kinetic energy of the system is given by the following expressions:

 Initial Momentum mv + mv or 2mv Initial Kinetic Energy 0.5*mv2 + 0.5*mv2 or mv2

If one ball (with mass m) popped out with twice the velocity (2v) then the final momentum and kinetic energy would be given by the following expressions:

 Final Momentum m*2v or 2mv (same as initial momentum) Initial Kinetic Energy 0.5*m*(2v)2 = 0.5*m*(4v2) or 2mv2 (this indicates a gain in KE)

Kinetic Energy (4 seconds) | Potential Energy (12 seconds) | Mechanical Energy (15 seconds)

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23. A 2 kg mass has a velocity of 4 m/s. The kinetic energy of the mass is ___ Joules.
 a. 4 b. 8 c. 16 d. 32 e. none of these

This is a relatively simple plug-and-chug into the equation KE=0.5*m*v2 with m=2 kg and v=4 m/s.

Kinetic Energy (4 seconds)

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24. A car moving at 50 km/hr skids 20 meters with locked brakes. How far will the car skid with locked brakes if it is traveling at 150 km/hr?
 a. 20 m b. 60 m c. 90 m d. 120 m e. 180 m

Recall the reasoning from the stopping distance-speed lab (with the Hot Wheels car and the computer box) that the stopping distance is related to the square of the initial speed. So any modification in the initial speed of a skidding car will lead to a square of that same modification in the stopping distance. A change in speed from 50 km/hr to 150 km/hr is a 3-fold increase. The distance in turn must change by nine-fold (3^2). So take the original stopping distance of 20 m and multiply it by 9.

Analysis of Situations Involving External Forces (21 seconds)

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25. A 50 kg diver hits the water below (at a zero height) with a kinetic energy of 5000 Joules. The height from which the diver dove was ____ meters.
 a. 5 b. 10 c. 50 d. 100

The kinetic energy of the diver must be equal to the original potential energy. Thus,

m*g*hi = KEf

(50 kg)*(10 m/s/s)*h = 5000 J

So, h = 10 m

Mechanical Energy Conservation (13 seconds)

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26. A job is done slowly, and an identical job is done quickly. Both jobs require the same amount of work, but different amounts of ___________.
 a. energy b. power c. both of these d. none of these

Power refers to the rate at which work is done. Thus, doing two jobs - one slowly and one quickly - involves doing the same job (i.e., the same work) at different rates or with different power.

Power (13 seconds)

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27. Which requires more work: lifting a 50 kg sack vertically 2 meters or lifting a 25 kg sack vertically 4 meters?
 a. lifting the 50 kg sack b. lifting the 25 kg sack c. both require the same amount of work

Work involves a force acting upon an object to cause a displacement. The amount of work done is found by multiplying F*d*cos(Theta). The equation can be used for these two motions to find the work.

 Lifting a 50 kg sack vertically 2 meters W = (500 N)*(2 m)*cos(0) W = 1000 N (Note: The weight of a 50-kg object is approx. 500 N; it takes 500 N to lift the object up.) Lifting a 25 kg sack vertically 4 meters W = (250 N)*(4 m)*cos(0) W = 1000 N (Note: The weight of a 50-kg object is approx. 250 N; it takes 250 N to lift the object up.)

Definition and Mathematics of Work (10 seconds) | Calculating the Amount of Work Done by Forces (10 seconds)

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28. A 50 kg sack is lifted 2 meters in the same time as a 25 kg sack is lifted 4 meters. The power expended in raising the 50 kg sack compared to the power used to lift the 25 kg sack is _________.
 a. twice as much b. half as much c. the same

The power is the rate at which work is done. Power is found by dividing work by time. It requires the same amount of work to do these two jobs (see question #27) and the same amount of time. Thus, the power is the same for both tasks.

Power (13 seconds)

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29. A TV set is pushed a distance of 2 m with a force of 20 N that is in the same direction as the set moves. How much work is done on the set?
 a. 2 J b. 10 J c. 20 J d. 40 J e. 80 J

This is a relatively simple plug-and-chug into the equation W=F*d*cos(Theta) with F=20 N and d=2 m and Theta = 0 degrees.

Calculating the Amount of Work Done by Forces (10 seconds)

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30. It takes 40 J to push a large box 4 m across a floor. Assuming the push is in the same direction as the move, what is the magnitude of the force on the box?
 a. 4 N b. 10 N c. 40 N d. 160 N e. none of these

This is a relatively simple plug-and-chug into the equation W=F*d*cos(Theta) with W=40 J and d=4 m and Theta = 0 degrees.

Calculating the Amount of Work Done by Forces (10 seconds)

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31. Using 1000 J of work, a toy elevator is raised from the ground floor to the second floor in 20 seconds. How much power does the elevator use?
 a. 20 W b. 50 W c. 100 W d. 1000 W e. 20000 W

This is a relatively simple plug-and-chug into the equation P=W/t with W=1000 J and t=20 s.

Calculating the Amount of Work Done by Forces (10 seconds) | Power (13 seconds)

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