Q: A 10 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 maximum force that can be applied horizontally before the box starts to move?

$$f_{s,\text{max}} = \mu_s n = 0.4 \times (10 \times 9.8) = 39.2\ \text{N}$$

Q: A car of mass 1200 kg accelerates from rest under a net force of 3600 N. What is the acceleration of the car?

$$a = \frac{F_{\text{net}}}{m} = \frac{3600}{1200} = 3\ \text{m/s}^2$$

Q: A 5 kg block slides on a horizontal surface with a coefficient of kinetic friction $$\mu_k = 0.2. $$What is the magnitude of the frictional force acting on the block?

$$f_k = \mu_k n = 0.2 \times (5 \times 9.8) = 9.8\ \text{N}$$

Q: A person pushes a 2 kg box with a force of 15 N to the right. The frictional force opposing the motion is 5 N. What is the acceleration of the box?

$$a = \frac{F_{\text{net}}}{m} = \frac{15 - 5}{2} = 5\ \text{m/s}^2$$

Q: A 50 kg crate is pulled with a horizontal force of 200 N. The coefficient of kinetic friction is 0.3. What is the acceleration of the crate?

$$a = \frac{F_{\text{applied}} - f_k}{m} = \frac{200 - (0.3 \times 50 \times 9.8)}{50} = \frac{200 - 147}{50} = 1.06\ \text{m/s}^2$$

Q: A 4 kg object is acted on by two forces: 10 N to the right and 6 N to the left. What is the acceleration of the object?

$$a = \frac{10 - 6}{4} = 1\ \text{m/s}^2$$\ \text{to the right}

Q: A 2 kg block is at rest on a table. The coefficient of static friction is 0.5. What is the minimum horizontal force required to start moving the block?

$$f_{s,\text{max}} = \mu_s n = 0.5 \times (2 \times 9.8) = 9.8\ \text{N}$$

Question 1

A 10 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 maximum force that can be applied horizontally before the box starts to move?

Accepted Answer

$$f_{s,	ext{max}} = \mu_s n = 0.4 	imes (10 	imes 9.8) = 39.2\ 	ext{N}$$

Question 2

A car of mass 1200 kg accelerates from rest under a net force of 3600 N. What is the acceleration of the car?

Accepted Answer

$$a = \frac{F_{	ext{net}}}{m} = \frac{3600}{1200} = 3\ 	ext{m/s}^2$$

Question 3

A 5 kg block slides on a horizontal surface with a coefficient of kinetic friction $$\mu_k = 0.2. $$What is the magnitude of the frictional force acting on the block?

Accepted Answer

$$f_k = \mu_k n = 0.2 	imes (5 	imes 9.8) = 9.8\ 	ext{N}$$

Question 4

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

Accepted Answer

An object at rest remains at rest and an object in motion remains in motion at constant velocity unless acted upon by a net external force.

Question 5

A person pushes a 2 kg box with a force of 15 N to the right. The frictional force opposing the motion is 5 N. What is the acceleration of the box?

Accepted Answer

$$a = \frac{F_{	ext{net}}}{m} = \frac{15 - 5}{2} = 5\ 	ext{m/s}^2$$

Question 6

If a book rests on a table and the table exerts an upward normal force on the book, according to Newton’s Third Law, what is the reaction force?

Accepted Answer

The book exerts an equal and opposite force downward on the table.

Question 7

A 3 kg object is moving at constant velocity on a horizontal surface. What can you conclude about the net force acting on the object?

Accepted Answer

$$F_{	ext{net}} = 0$$

Question 8

A 50 kg crate is pulled with a horizontal force of 200 N. The coefficient of kinetic friction is 0.3. What is the acceleration of the crate?

Accepted Answer

$$a = \frac{F_{	ext{applied}} - f_k}{m} = \frac{200 - (0.3 	imes 50 	imes 9.8)}{50} = \frac{200 - 147}{50} = 1.06\ 	ext{m/s}^2$$

Question 9

Which equation correctly represents the maximum static friction force?

Accepted Answer

$$f_{s,	ext{max}} = \mu_s n$$

Question 10

A hockey puck slides across ice with negligible friction. According to Newton’s First Law, what will happen to the puck if no external forces act on it?

Accepted Answer

It will continue moving at constant velocity.

Question 11

A 4 kg object is acted on by two forces: 10 N to the right and 6 N to the left. What is the acceleration of the object?

Accepted Answer

$$a = \frac{10 - 6}{4} = 1\ 	ext{m/s}^2$$\ 	ext{to the right}

Question 12

A 2 kg block is at rest on a table. The coefficient of static friction is 0.5. What is the minimum horizontal force required to start moving the block?

Accepted Answer

$$f_{s,	ext{max}} = \mu_s n = 0.5 	imes (2 	imes 9.8) = 9.8\ 	ext{N}$$

Question 13

A person stands on a scale in an elevator accelerating upward at $$2\ 	ext{m/s}^2. If $$the person’s mass is 70 kg, what is the normal force exerted by the scale?

Accepted Answer

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

Question 14

A 1 kg object is pushed with a force of 5 N on a frictionless surface. What is its acceleration?

Accepted Answer

$$a = \frac{F_{	ext{net}}}{m} = \frac{5}{1} = 5\ 	ext{m/s}^2$$

Question 15

A 20 kg sled is pulled across snow with a constant velocity. The coefficient of kinetic friction is 0.1. What is the force of kinetic friction?

Accepted Answer

$$f_k = \mu_k n = 0.1 	imes (20 	imes 9.8) = 19.6\ 	ext{N}$$

Question 16

A 15 kg box is at rest on a horizontal floor. What is the normal force acting on the box?

Accepted Answer

$$n = m g = 15 	imes 9.8 = 147\ 	ext{N}$$

Newton's Laws of Motion and Friction Forces: Study Notes

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