MCAS Power Practice

1.

The motor of one car is more powerful than the motor of another car. Which of the following must be true of the more powerful motor?

A.

It can do work more quickly
B.

It can operate for a longer time
C.

It can burn fuel more efficiently
D.

It can store more potential energy

Correct Answer
A. It can do work more quickly

Explanation
Because power is the amount of work done in a given period of time (P=W/t), if a motor is more powerful, more work is done in a shorter period of time.

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2.

A crane lifts two 100 kg loads of building materials to the top of a building under construction. The first load is lifted in 10 s and the second load is lifted in 13 s. Which of the following statements BEST compares the work and power expended by the crane lifting the two loads?

A.

The crane does more work and expends more power for the first lift.
B.

The crane does the same work and expends the same power for each lift.
C.

The crane does the same work for each lift but expends more power for the first lift.
D.

The crane does more work in the second lift but expends the same power for each lift.

Correct Answer
C. The crane does the same work for each lift but expends more power for the first lift.

Explanation
Work is a force exerted to move an object a distance (W=Fd). Because the crane lifts the same load (100 kg) the same distance (to the top of a building), the same amount of work is done in both lifts. Power is the amount of work done in a period of time. The crane does the work in a shorter period of time lifting the first load (10 s), therefore the crane expends more power doing that work in a shorter amount of time.

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3.

Two students each carry a box up to the third floor of a building. The total mass of each student and the box she is carrying is the same. Roberta makes the trip in 45 s while Mary takes 30 s. Which of the following statements BEST describes this task?

A.

Roberta does more total work than Mary
B.

Mary does more total work than Roberta
C.

Roberta has greater power output than Mary
D.

Mary has a greater power output than Roberta

Correct Answer
D. Mary has a greater power output than Roberta

Explanation
Work is a force exerted to move an object a distance (W=Fd). Because the students lift the same load (same weight, same box) the same distance (to the third floor), the same amount of work is done by both girls. Power is the amount of work done in a period of time. Mary does the work in a shorter period of time carrying the box to the third floor in 30 s, while Roberta takes 45 s. Therefore Mary has a greater power output than Roberta.

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4.

A man ran on a treadmill for 1800 seconds. At the end of his run, the treadmill indicated his energy output as 240,000 J. What average power did he generate?

A.

666 W
B.

133 W
C.

66 W
D.

7 W

Correct Answer
B. 133 W

Explanation
Power is the amount of work done (or the amount of energy used) in a period of time (P=W/t). The work done (or energy used) was 240,000 J. The time was 1800 s. Therefore, P = 240,000 J / 1800 s = 133.33 W

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5.

At a weightlifting competition, two competitors lifted the same weight to the same height. The second competitor accomplished the lift 2 seconds faster than the first competitor. This demonstrated that the second competitor had more

A.

Energy than the first
B.

Inertia than the first
C.

Power than the first
D.

Work than the first

Correct Answer
C. Power than the first

Explanation
Work is a force exerted to move an object a distance (W=Fd). Because each competitor lifts the same weight to the same height (distance), the same amount of work is done by both competitors. Power is the amount of work done in a period of time. The first second competitor does the work in a shorter period of time lifting the weight 2 s faster than the first competitor. Therefore the second competitor has more power than the first.

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