Q: A person pulls a wagon with a force of 50 N at an angle of 30° to the horizontal for a distance of 30 m. What is the work done by the force in moving the wagon?

$$W = 50 \times 30 \times \cos(30^\circ) = 1299\ \text{J}$$

Q: A weightlifter lifts a 100 kg barbell to a height of 2 m. What is the increase in gravitational potential energy of the barbell? (Take $$g = 9.8\ \text{m/s}^2)$$

$$PE = 100 \times 9.8 \times 2 = 1960\ \text{J}$$

Q: A car of mass 1000 kg increases its speed from 10 m/s to 20 m/s. What is the change in its kinetic energy?

$$\Delta KE = \frac{1}{2} \times 1000 \times (20^2$ - 10^2$) = 150,000\ \text{J}$$

Q: A 10-N ball is lifted vertically by 3 m. What is the work done against gravity?

$$W = 10 \times 3 = 30\ \text{J}$$

Q: A runner of mass 60 kg is moving at 8 m/s. What is her kinetic energy?

$$KE = \frac{1}{2} \times 60 \times 8^2$ = 1920\ \text{J}$$

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

$$P = 500 / 10 = 50\ \text{W}$$

Question 1

A person pulls a wagon with a force of 50 N at an angle of 30° to the horizontal for a distance of 30 m. What is the work done by the force in moving the wagon?

Accepted Answer

$$W = 50 	imes 30 	imes \cos(30^\circ) = 1299\ 	ext{J}$$

Question 2

A weightlifter lifts a 100 kg barbell to a height of 2 m. What is the increase in gravitational potential energy of the barbell? (Take $$g = 9.8\ 	ext{m/s}^2)$$

Accepted Answer

$$PE = 100 	imes 9.8 	imes 2 = 1960\ 	ext{J}$$

Question 3

Which of the following statements about work is correct?

Accepted Answer

Work can be positive or negative depending on the direction of force and displacement.

Question 4

A car of mass 1000 kg increases its speed from 10 m/s to 20 m/s. What is the change in its kinetic energy?

Accepted Answer

$$\Delta KE = \frac{1}{2} 	imes 1000 	imes (20^2$ - 10^2$) = 150,000\ 	ext{J}$$

Question 5

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

Accepted Answer

The work is equal to the force times the distance moved by the wall, which is zero.

Question 6

A 10-N ball is lifted vertically by 3 m. What is the work done against gravity?

Accepted Answer

$$W = 10 	imes 3 = 30\ 	ext{J}$$

Question 7

A runner of mass 60 kg is moving at 8 m/s. What is her kinetic energy?

Accepted Answer

$$KE = \frac{1}{2} 	imes 60 	imes 8^2$ = 1920\ 	ext{J}$$

Question 8

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

Accepted Answer

Energy cannot be created or destroyed; it can only be transformed from one form to another.

Question 9

A bow stores 50 J of potential energy when drawn. When the arrow is released, it leaves the bow with 40 J of kinetic energy. What happens to the remaining energy?

Accepted Answer

10 J is transformed into heat in the bow.

Question 10

A car is hoisted for repairs in a service station. Compared to its potential energy on the ground, its potential energy relative to the floor is:

Accepted Answer

Increased

Question 11

Which of the following is the correct unit for power?

Accepted Answer

Watt

Question 12

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

Accepted Answer

$$P = 500 / 10 = 50\ 	ext{W}$$

Question 13

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

Accepted Answer

Kinetic energy is a scalar, momentum is a vector.

Question 14

A 2 kg object is dropped from a height of 5 m. What is its gravitational potential energy at the top? (Take $$g = 9.8\ 	ext{m/s}^2)$$

Accepted Answer

$$PE = 2 	imes 9.8 	imes 5 = 98\ 	ext{J}$$

Question 15

A car engine delivers twice the power of another engine. Which of the following is true if both engines do the same amount of work?

Accepted Answer

The more powerful engine does the work in half the time.

Question 16

Which equation correctly expresses the work-energy theorem?

Accepted Answer

$$W_{total} = \Delta K = \frac{1}{2}mv_f^2$ - \frac{1}{2}mv_i^2$

Question 17

A person pushes a cart with a constant force. If the distance is doubled, what happens to the work done?

Accepted Answer

It is twice as much.

Question 18

Which of the following is an example of work done against a force?

Accepted Answer

A person lifting a box.

Question 19

If the speed of an object is doubled, what happens to its kinetic energy?

Accepted Answer

It quadruples.

Question 20

A 60 W light bulb is left on for 5 hours. How much energy does it consume in kilowatt-hours?

Accepted Answer

$$E = 0.06 	imes 5 = 0.3\ 	ext{kWh}$$

Chapter 7: Energy – Physics Study Notes

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