Q: A 10 kg box rests on a horizontal surface. If the coefficient of static friction between the box and the surface is 0.4, what is the maximum horizontal force that can be applied to the box before it starts to move?

$$F_{\text{max}} = \mu_s n = 0.4 \times 10 \times 9.8 = 39.2\ \text{N}$$

Q: A passenger with mass $$70\ \text{kg}$$ stands on a scale in an elevator accelerating upward at $$2\ \text{m/s}^2. $$What does the scale read?

$$n = m(g + a_y) = 70 \times (9.8 + 2) = 826\ \text{N}$$

Q: A car rounds a flat curve of radius $$50\ \text{m} at a $$speed of $$20\ \text{m/s}. $$What is the minimum coefficient of static friction required to prevent skidding?

$$\mu_s = \frac{v^2$}{rg} = \frac{20^2$}{50 \times 9.8} = 0.82$$

Q: A skydiver of mass $$80\ \text{kg}$$ reaches terminal velocity during free fall. If the drag force is proportional to velocity, what is the magnitude of the drag force at terminal velocity?

$$F_{\text{drag}} = mg = 80 \times 9.8 = 784\ \text{N}$$

Q: A block is pulled at constant speed across a horizontal surface. Which statement is true about the forces acting on the block?

The applied force equals the kinetic friction force: $$F_{\text{applied}} = f_k$$

Q: A $$5\ \text{kg}$$ object is acted on by a net force of $$15\ \text{N} to $$the right. What is its acceleration?

$$a = \frac{F}{m} = \frac{15}{5} = 3\ \text{m/s}^2$$

Q: A car is rounding a banked curve of radius $$100\ \text{m} at a $$speed of $$25\ \text{m/s}. At $$what angle $$\beta$$ should the road be banked so that no friction is required?

$$\tan \beta = \frac{v^2$}{rg} = \frac{25^2$}{100 \times 9.8} = 0.64 \implies \beta = \arctan(0.64) \approx 32.7^\circ$$

Question 1

A 10 kg box rests on a horizontal surface. If the coefficient of static friction between the box and the surface is 0.4, what is the maximum horizontal force that can be applied to the box before it starts to move?

Accepted Answer

$$F_{	ext{max}} = \mu_s n = 0.4 	imes 10 	imes 9.8 = 39.2\ 	ext{N}$$

Question 2

A passenger with mass $$70\ 	ext{kg}$$ stands on a scale in an elevator accelerating upward at $$2\ 	ext{m/s}^2. $$What does the scale read?

Accepted Answer

$$n = m(g + a_y) = 70 	imes (9.8 + 2) = 826\ 	ext{N}$$

Question 3

Which of the following statements about free-body diagrams is correct?

Accepted Answer

The acceleration vector $$\vec{a}$$ can be drawn to the side, but $$m\vec{a}$$ should not be included as a force.

Question 4

A car rounds a flat curve of radius $$50\ 	ext{m} at a $$speed of $$20\ 	ext{m/s}. $$What is the minimum coefficient of static friction required to prevent skidding?

Accepted Answer

$$\mu_s = \frac{v^2$}{rg} = \frac{20^2$}{50 	imes 9.8} = 0.82$$

Question 5

A skydiver of mass $$80\ 	ext{kg}$$ reaches terminal velocity during free fall. If the drag force is proportional to velocity, what is the magnitude of the drag force at terminal velocity?

Accepted Answer

$$F_{	ext{drag}} = mg = 80 	imes 9.8 = 784\ 	ext{N}$$

Question 6

Which of the following is NOT one of the four fundamental forces of nature?

Accepted Answer

Frictional force

Question 7

A block is pulled at constant speed across a horizontal surface. Which statement is true about the forces acting on the block?

Accepted Answer

The applied force equals the kinetic friction force: $$F_{	ext{applied}} = f_k$$

Question 8

A $$5\ 	ext{kg}$$ object is acted on by a net force of $$15\ 	ext{N} to $$the right. What is its acceleration?

Accepted Answer

$$a = \frac{F}{m} = \frac{15}{5} = 3\ 	ext{m/s}^2$$

Question 9

A car is rounding a banked curve of radius $$100\ 	ext{m} at a $$speed of $$25\ 	ext{m/s}. At $$what angle $$\beta$$ should the road be banked so that no friction is required?

Accepted Answer

$$	an \beta = \frac{v^2$}{rg} = \frac{25^2$}{100 	imes 9.8} = 0.64 \implies \beta = \arctan(0.64) \approx 32.7^\circ$$

Question 10

A box of mass $$m is at $$rest on a horizontal surface. A horizontal force $$F is $$applied, but the box does not move. Which equation correctly describes the static friction force $$f_s$$?

Accepted Answer

$$f_s = F$$

Question 11

A block slides down a rough incline at constant speed. Which statement is true?

Accepted Answer

The component of gravity down the incline equals the kinetic friction force.

Question 12

Which of the following best describes the condition for an object to be in equilibrium?

Accepted Answer

$$\sum F_x = 0$$ and $$\sum F_y = 0$$

Question 13

A $$2\ 	ext{kg}$$ object is dropped from rest and falls through air. When the drag force equals half its weight, what is the acceleration?

Accepted Answer

$$a = g - \frac{F_{	ext{drag}}}{m} = 9.8 - \frac{9.8}{2} = 4.9\ 	ext{m/s}^2$$

Question 14

Which of the following is a correct application of Newton’s third law?

Accepted Answer

A book pushes down on a table with force $$F$$, and the table pushes up on the book with force $$F.$$

Question 15

A block is at rest on a horizontal surface. Which of the following statements about the normal force $$n is $$correct?

Accepted Answer

$$n = mg$$

Question 16

A car is moving in a circle of radius $$r at $$constant speed $$v. $$What is the direction of the net force acting on the car?

Accepted Answer

Toward the center of the circle

Ch 5: Applying Newton’s Laws: Equilibrium, Dynamics, Friction, Circular Motion, and Fundamental Forces

Study Guide - Practice Questions

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