Q: A 2 kg book is lifted from the ground to a shelf 3 meters high. What is the change in gravitational potential energy of the book? (Assume $$g = 9.8 \ \mathrm{m/s^2$})$$

$$PE = mgh = 2 \times 9.8 \times 3 = 58.8 \ \mathrm{J}$$

Q: A 0.5 kg ball is dropped from a height of 10 m. What is its speed just before it hits the ground? (Assume $$g = 9.8 \ \mathrm{m/s^2$})$$

$$v = \sqrt{2gh} = \sqrt{2 \times 9.8 \times 10} = 14 \ \mathrm{m/s}$$

Q: A person pushes a box with a force of 50 N over a distance of 4 m. How much work is done on the box?

$$W = Fd = 50 \times 4 = 200 \ \mathrm{J}$$

Q: A 60 kg person runs up a flight of stairs 5 meters high in 4 seconds. What is the average power output? (Assume $$g = 9.8 \ \mathrm{m/s^2$})$$

$$P = \frac{mgh}{t} = \frac{60 \times 9.8 \times 5}{4} = 735 \ \mathrm{W}$$

Question 1

A 2 kg book is lifted from the ground to a shelf 3 meters high. What is the change in gravitational potential energy of the book? (Assume $$g = 9.8 \ \mathrm{m/s^2$})$$

Accepted Answer

$$PE = mgh = 2 	imes 9.8 	imes 3 = 58.8 \ \mathrm{J}$$

Question 2

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

Accepted Answer

Energy is never created or destroyed, only transformed from one form to another.

Question 3

A 0.5 kg ball is dropped from a height of 10 m. What is its speed just before it hits the ground? (Assume $$g = 9.8 \ \mathrm{m/s^2$})$$

Accepted Answer

$$v = \sqrt{2gh} = \sqrt{2 	imes 9.8 	imes 10} = 14 \ \mathrm{m/s}$$

Question 4

Which of the following forces does NOT contribute to potential energy?

Accepted Answer

Frictional force

Question 5

A person pushes a box with a force of 50 N over a distance of 4 m. How much work is done on the box?

Accepted Answer

$$W = Fd = 50 	imes 4 = 200 \ \mathrm{J}$$

Question 6

A wind turbine converts the kinetic energy of wind into electrical energy. If the wind speed doubles, by what factor does the kinetic energy of the wind increase?

Accepted Answer

It increases by a factor of 4

Question 7

A 60 kg person runs up a flight of stairs 5 meters high in 4 seconds. What is the average power output? (Assume $$g = 9.8 \ \mathrm{m/s^2$})$$

Accepted Answer

$$P = \frac{mgh}{t} = \frac{60 	imes 9.8 	imes 5}{4} = 735 \ \mathrm{W}$$

Question 8

Which of the following scenarios involves zero work being done?

Accepted Answer

A person holds a box stationary at arm's length.

Question 9

A pendulum is released from rest at a certain height. At the lowest point of its swing, which statement is true?

Accepted Answer

Kinetic energy is maximum, potential energy is zero.

Question 10

A demolition ball is held at a height, then released to strike a building. Which sequence correctly describes the energy transformations?

Accepted Answer

Potential energy → kinetic energy → work

Question 11

A machine does 500 J of work in 10 seconds. What is its power output?

Accepted Answer

$$P = \frac{W}{t} = \frac{500}{10} = 50 \ \mathrm{W}$$

Question 12

Which of the following is a correct unit for work?

Accepted Answer

Newton-meter

Question 13

A person pushes a box across a rough floor at constant speed. Which statement is true about the energy transformations?

Accepted Answer

Most work done is converted into thermal energy due to friction.

Question 14

A simple machine allows you to lift a 200 N load by applying a force of only 50 N. If the load is lifted 1 m, how far must you apply the force (assuming 100% efficiency)?

Accepted Answer

$$d_{in} = \frac{F_{out} d_{out}}{F_{in}} = \frac{200 	imes 1}{50} = 4 \ \mathrm{m}$$

Question 15

Which of the following best explains why not all energy in a system is available to do useful work?

Accepted Answer

Some energy is always converted to thermal energy, which is less useful.

Question 16

A 1000 kg car is moving at 20 m/s. What is its kinetic energy?

Accepted Answer

$$KE = \frac{1}{2}mv^2$ = \frac{1}{2} 	imes 1000 	imes 20^2$ = 200,000 \ \mathrm{J}$$

Work, Energy, and Power – Physics Chapter 7 Study Notes

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