Q: A 5 kg box is at rest on a horizontal surface. The coefficient of static friction between the box and the surface is 0.4. What is the minimum horizontal force required to start moving the box?

$$F = \mu_s F_N = 0.4 \times 5 \times 9.8 = 19.6\ \mathrm{N}$$

Q: A 2 kg block slides down a frictionless incline at $$30^\circ to $$the horizontal. What is the acceleration of the block down the incline?

$$a = g \sin 30^\circ = 9.8 \times 0.5 = 4.9\ \mathrm{m/s^2$}$$

Q: A 1000 kg elevator is being accelerated upward at $$2\ \mathrm{m/s^2$}. $$What is the tension in the cable pulling the elevator up?

$$T = m (g + a) = 1000 \times (9.8 + 2) = 11,800\ \mathrm{N}$$

Q: A 5 kg box is moving at $$4\ \mathrm{m/s}$$ with a coefficient of kinetic friction $$\mu_k = 0.25. $$How far will the box slide before coming to rest?

$$d = \frac{v^2$}{2 \mu_k g} = \frac{16}{2 \times 0.25 \times 9.8} = 3.27\ \mathrm{m}$$

Question 1

Which of the following statements best describes Newton's First Law of Motion?

Accepted Answer

An object at rest will remain at rest, and an object in motion will remain in motion with constant velocity unless acted upon by an outside force.

Question 2

A 5 kg box is at rest on a horizontal surface. The coefficient of static friction between the box and the surface is 0.4. What is the minimum horizontal force required to start moving the box?

Accepted Answer

$$F = \mu_s F_N = 0.4 	imes 5 	imes 9.8 = 19.6\ \mathrm{N}$$

Question 3

A 2 kg block slides down a frictionless incline at $$30^\circ to $$the horizontal. What is the acceleration of the block down the incline?

Accepted Answer

$$a = g \sin 30^\circ = 9.8 	imes 0.5 = 4.9\ \mathrm{m/s^2$}$$

Question 4

Which of the following is the correct expression for the force of kinetic friction?

Accepted Answer

$$f_k = \mu_k F_N$$

Question 5

A 1000 kg elevator is being accelerated upward at $$2\ \mathrm{m/s^2$}. $$What is the tension in the cable pulling the elevator up?

Accepted Answer

$$T = m (g + a) = 1000 	imes (9.8 + 2) = 11,800\ \mathrm{N}$$

Question 6

A 5 kg block has two forces acting on it: $$15\ \mathrm{N} to $$the north and $$20\ \mathrm{N} to $$the west. What is the magnitude of the block's acceleration?

Accepted Answer

$$a = \sqrt{15^2$ + 20^2$} / 5 = 5\ \mathrm{m/s^2$}$$

Question 7

Which of the following statements about Newton's Third Law is correct?

Accepted Answer

Action and reaction forces are always equal in magnitude and opposite in direction.

Question 8

A 10 kg painting is hanging from two wires, each making a $$45^\circ$$ angle with the vertical. What is the tension in each wire?

Accepted Answer

$$T = \frac{mg}{2 \sin 45^\circ} = \frac{98}{1.414} = 69.3\ \mathrm{N}$$

Question 9

A 5 kg box is moving at $$4\ \mathrm{m/s}$$ with a coefficient of kinetic friction $$\mu_k = 0.25. $$How far will the box slide before coming to rest?

Accepted Answer

$$d = \frac{v^2$}{2 \mu_k g} = \frac{16}{2 	imes 0.25 	imes 9.8} = 3.27\ \mathrm{m}$$

Question 10

A 20 kg lawn mower is pushed with a force of $$100\ \mathrm{N} at a$$ $$45^\circ$$ angle to the horizontal. What is the normal force on the mower?

Accepted Answer

$$F_N = mg - 100 \sin 45^\circ = 196 - 70.7 = 125.3\ \mathrm{N}$$

Question 11

Which of the following best describes inertia?

Accepted Answer

The tendency of an object to maintain its state of motion.

Question 12

A 5 kg box is on a $$30^\circ$$ incline with a coefficient of kinetic friction $$\mu_k = 0.2. $$What is the acceleration of the box down the incline?

Accepted Answer

$$a = g (\sin 30^\circ - \mu_k \cos 30^\circ) = 9.8 (0.5 - 0.2 	imes 0.866) = 3.8\ \mathrm{m/s^2$}$$

Question 13

Which of the following is true about the normal force?

Accepted Answer

It is always perpendicular to the surface.

Question 14

A spring has a spring constant $$k = 10\ \mathrm{N/m}. $$You hang a $$2\ \mathrm{kg}$$ block from it. How far does the spring stretch when the block is in equilibrium?

Accepted Answer

$$\Delta x = \frac{mg}{k} = \frac{2 	imes 9.8}{10} = 1.96\ \mathrm{m}$$

Question 15

A box is being pushed horizontally against a wall. What is the minimum push necessary for the box not to slide, given the coefficient of static friction $$\mu_s$$ and mass $$m$$?

Accepted Answer

$$P = \frac{m g}{\mu_s}$$

Question 16

Which of the following is the correct form of Hooke's Law for a spring?

Accepted Answer

$$F_{	ext{spring}} = -k \Delta x$$

Question 17

You push with a $$6\ \mathrm{N}$$ force on a $$4\ \mathrm{kg}$$ box, which pushes on a $$2\ \mathrm{kg}$$ box. What is the acceleration of the system?

Accepted Answer

$$a = \frac{6}{6} = 1\ \mathrm{m/s^2$}$$

Question 18

Which of the following best explains why action-reaction forces do not cancel each other out?

Accepted Answer

They act on different objects.

Newton's Laws of Motion and Applications: Study Notes

스터디 가이드 - 연습 문제

학습 가이드 - 플래시카드