Q: A worker lifts a 20 kg box to a shelf 2 meters high. What is the gravitational potential energy gained by the box? (Assume $$g = 9.8 \mathrm{\ m/s^2$})$$

$$E_p = mgh = 20 \times 9.8 \times 2 = 392 \mathrm{\ J}$$

Q: You push a cart with a constant force of 50 N for 10 meters. How much work do you do?

$$W = Fd = 50 \times 10 = 500 \mathrm{\ J}$$

Q: A 10-N ball is lifted 3 meters. How much work is done against gravity?

$$W = Fd = 10 \times 3 = 30 \mathrm{\ J}$$

Question 1

Which of the following best describes the law of conservation of energy?

Accepted Answer

Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy in the universe never changes.

Question 2

A worker lifts a 20 kg box to a shelf 2 meters high. What is the gravitational potential energy gained by the box? (Assume $$g = 9.8 \mathrm{\ m/s^2$})$$

Accepted Answer

$$E_p = mgh = 20 	imes 9.8 	imes 2 = 392 \mathrm{\ J}$$

Question 3

Which equation correctly expresses the work-energy theorem?

Accepted Answer

$$W = \Delta KE$$

Question 4

A car traveling at 20 m/s has a kinetic energy of $$40,000 \mathrm{\ J}. If $$its speed is doubled, what is its new kinetic energy?

Accepted Answer

$$KE = 160,000 \mathrm{\ J}$$

Question 5

A machine is 30% efficient. If you input 1000 J of energy, how much is converted to useful work?

Accepted Answer

300 J

Question 6

You push a cart with a constant force of 50 N for 10 meters. How much work do you do?

Accepted Answer

$$W = Fd = 50 	imes 10 = 500 \mathrm{\ J}$$

Question 7

A 10-N ball is lifted 3 meters. How much work is done against gravity?

Accepted Answer

$$W = Fd = 10 	imes 3 = 30 \mathrm{\ J}$$

Question 8

Which of the following statements about kinetic energy and momentum is correct?

Accepted Answer

Momentum is a vector quantity and can be canceled; kinetic energy is a scalar and cannot be canceled.

Question 9

A woman uses an ideal pulley system to lift a 100-N crate by pulling down with a force of 25 N. For every 1 meter of rope she pulls, how far does the crate rise?

Accepted Answer

0.25 meters

Question 10

If the initial speed of a car is doubled, how does the stopping distance change, assuming the same friction force?

Accepted Answer

It quadruples.

Question 11

Which of the following is NOT a source of energy mentioned in the study materials?

Accepted Answer

Magnetism

Question 12

A fuel cell produces electricity by combining hydrogen and oxygen. What is the main product besides electricity?

Accepted Answer

Water

Question 13

Which formula gives the kinetic energy of an object of mass $$m$$ moving at speed $$v$$?

Accepted Answer

$$KE = \frac{1}{2}mv^2$

Question 14

A lever allows a small force over a large distance to induce a large force over a short distance. Which principle does this illustrate?

Accepted Answer

Conservation of energy

Question 15

If you push against a stationary brick wall for several minutes, what is true about the work done on the wall?

Accepted Answer

No work is done on the wall.

Question 16

A power plant wastes about 30% of its energy as heat. What is one way to improve its efficiency?

Accepted Answer

Recycle the waste heat to heat nearby buildings.

Question 17

Which equation correctly relates power, work, and time?

Accepted Answer

$$P = \frac{W}{t}$$

Question 18

A runner does 600 J of work in 10 seconds. What is her average power output?

Accepted Answer

$$P = \frac{600}{10} = 60 \mathrm{\ W}$$

Question 19

Which of the following statements about machines is correct?

Accepted Answer

Machines cannot multiply work or energy, only transform or transfer it.

Question 20

In a real-world scenario, why does the kinetic energy of a shot arrow not equal the potential energy stored in a drawn bow?

Accepted Answer

Some energy is transformed into heat in the bow and surroundings.

Chapter 7: Energy – Conceptual Physics Study Notes

스터디 가이드 - 연습 문제

학습 가이드 - 플래시카드