Q: A 5 kg box moves to the right with a speed of 3 m/s. What is its kinetic energy?

$$K = \frac{1}{2}mv^2$ = \frac{1}{2} \times 5 \times 3^2$ = 22.5\ \text{J}$$

Q: You pull a 10 kg suitcase with a 50 N force angled 37° above the floor for 20 m. What is the work done by your pull?

$$W = Fd\cos\theta = 50 \times 20 \times \cos 37^\circ = 798\ \text{J}$$

Q: A 75 kg hiker climbs a 1,000 m mountain. What is the work done by gravity during the hike?

$$W_g = -mg\Delta y = -75 \times 9.8 \times 1000 = -735,000\ \text{J}$$

Q: A 4 kg block moving at 20 m/s strikes a massless spring with force constant 600 N/m. What is the maximum compression of the spring?

$$\Delta x = \sqrt{\frac{mv^2$}{k}} = \sqrt{\frac{4 \times 20^2$}{600}} = 0.365\ \text{m}$$

Q: What is the work-energy theorem?

$$W_{\text{net}} = \Delta K = K_f - K_i$$

Q: A 2 kg object is dropped from 80 m and reaches the floor with 30 m/s. What is the work done by air resistance?

$$W_{\text{air}} = K_f + U_f - K_i - U_i = \frac{1}{2} \times 2 \times 30^2$ + 0 - 0 - 2 \times 9.8 \times 80 = 900 - 1568 = -668\ \text{J}$$

Q: Which equation correctly represents the elastic potential energy stored in a spring?

$$U_{\text{el}} = \frac{1}{2} k x^2$

Q: A 4 kg block is attached to a spring (k = 500 N/m) and pushed with a force of 100 N. What is the compression distance of the spring?

$$x = \frac{F}{k} = \frac{100}{500} = 0.2\ \text{m}$$

Question 1

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

Accepted Answer

Energy can only be transferred between forms and cannot be created or destroyed.

Question 2

A 5 kg box moves to the right with a speed of 3 m/s. What is its kinetic energy?

Accepted Answer

$$K = \frac{1}{2}mv^2$ = \frac{1}{2} 	imes 5 	imes 3^2$ = 22.5\ 	ext{J}$$

Question 3

You pull a 10 kg suitcase with a 50 N force angled 37° above the floor for 20 m. What is the work done by your pull?

Accepted Answer

$$W = Fd\cos	heta = 50 	imes 20 	imes \cos 37^\circ = 798\ 	ext{J}$$

Question 4

A 75 kg hiker climbs a 1,000 m mountain. What is the work done by gravity during the hike?

Accepted Answer

$$W_g = -mg\Delta y = -75 	imes 9.8 	imes 1000 = -735,000\ 	ext{J}$$

Question 5

Which force is always considered non-conservative and always does negative work?

Accepted Answer

Friction

Question 6

A 4 kg block moving at 20 m/s strikes a massless spring with force constant 600 N/m. What is the maximum compression of the spring?

Accepted Answer

$$\Delta x = \sqrt{\frac{mv^2$}{k}} = \sqrt{\frac{4 	imes 20^2$}{600}} = 0.365\ 	ext{m}$$

Question 7

What is the work-energy theorem?

Accepted Answer

$$W_{	ext{net}} = \Delta K = K_f - K_i$$

Question 8

A 2 kg object is dropped from 80 m and reaches the floor with 30 m/s. What is the work done by air resistance?

Accepted Answer

$$W_{	ext{air}} = K_f + U_f - K_i - U_i = \frac{1}{2} 	imes 2 	imes 30^2$ + 0 - 0 - 2 	imes 9.8 	imes 80 = 900 - 1568 = -668\ 	ext{J}$$

Question 9

Which equation correctly represents the elastic potential energy stored in a spring?

Accepted Answer

$$U_{	ext{el}} = \frac{1}{2} k x^2$

Question 10

A 1000 kg elevator is lifted 100 m in 50 seconds at constant speed. What is the power supplied by the motor?

Accepted Answer

$$P = \frac{W}{\Delta t} = \frac{mg\Delta y}{\Delta t} = \frac{1000 	imes 9.8 	imes 100}{50} = 19,600\ 	ext{W}$$

Question 11

Which of the following is a conservative force?

Accepted Answer

Gravity

Question 12

A 4 kg block is attached to a spring (k = 500 N/m) and pushed with a force of 100 N. What is the compression distance of the spring?

Accepted Answer

$$x = \frac{F}{k} = \frac{100}{500} = 0.2\ 	ext{m}$$

Question 13

A 3 kg box is pulled horizontally with a force of 40 N at an angle of 53° for 2 m. What is the work done by gravity?

Accepted Answer

$$W_g = 0$$

Question 14

A 0.5 kg hockey puck is pushed with a constant 200 N force through a distance of 0.3 m. What is the final speed of the puck if it started at 4 m/s?

Accepted Answer

$$v_f = \sqrt{v_i^2$ + \frac{2W}{m}} = \sqrt{4^2$ + \frac{2 	imes 200 	imes 0.3}{0.5}} = \sqrt{16 + 240} = \sqrt{256} = 16\ 	ext{m/s}$$

Question 15

Which of the following best describes the mechanical energy of a system?

Accepted Answer

$$ME = K + U$$

Question 16

A 4 kg block moving on a flat surface strikes a spring (k = 600 N/m) with a coefficient of friction 0.5 and speed 20 m/s. What is the maximum compression of the spring?

Accepted Answer

$$\Delta x = \sqrt{\frac{mv^2$}{k} - \frac{f_k d}{k}}$$

Question 17

Which of the following is true about the work done by gravity on an object moving along an inclined plane?

Accepted Answer

It depends only on the change in vertical height, not the path taken.

Ch 10: Energy & Work – Study Notes

Guida di studio - Domande di pratica

Study guide - Flashcard