Q: A 20 kg box is pushed with a force of 150 N on a horizontal surface. If the coefficient of kinetic friction is $$\mu_k = 0.2$$, what is the frictional force acting on the box?

$$f_k = 0.2 \times 20 \times 9.8 = 39.2\ \mathrm{N}$$

Q: A person with a mass of 60 kg stands on an elevator that is moving upward at a constant velocity. What is the normal force exerted by the elevator floor on the person?

$$F_N = 60 \times 9.8 = 588\ \mathrm{N}$$

Q: A girl that weighs 500 N exerts a 300 N force on a box that weighs 1000 N. If the coefficient of static friction is $$\mu_s = 0.2$$, what is the maximum static friction force on the box?

$$f_{s,\text{max}} = 0.2 \times 1000 = 200\ \mathrm{N}$$

Q: A box of mass 10 kg is at rest on a horizontal surface. What is the normal force acting on the box?

$$F_N = 10 \times 9.8 = 98\ \mathrm{N}$$

Q: A 20 kg box is sliding along a horizontal surface with a coefficient of kinetic friction $$\mu_k = 0.2. If $$the box is pulled horizontally with a force of 150 N, what is the net force acting on the box?

$$F_{\text{net}} = 150 - (0.2 \times 20 \times 9.8) = 110.8\ \mathrm{N}$$

Q: A 5 kg object is suspended by a rope. What is the tension in the rope if the object is at rest?

$$T = 5 \times 9.8 = 49\ \mathrm{N}$$

Question 1

According to Newton's First Law of Motion, what will happen to a body at rest if no net force acts upon it?

Accepted Answer

It will remain at rest.

Question 2

A 20 kg box is pushed with a force of 150 N on a horizontal surface. If the coefficient of kinetic friction is $$\mu_k = 0.2$$, what is the frictional force acting on the box?

Accepted Answer

$$f_k = 0.2 	imes 20 	imes 9.8 = 39.2\ \mathrm{N}$$

Question 3

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

Accepted Answer

For every action, there is an equal and opposite reaction.

Question 4

A person with a mass of 60 kg stands on an elevator that is moving upward at a constant velocity. What is the normal force exerted by the elevator floor on the person?

Accepted Answer

$$F_N = 60 	imes 9.8 = 588\ \mathrm{N}$$

Question 5

A girl that weighs 500 N exerts a 300 N force on a box that weighs 1000 N. If the coefficient of static friction is $$\mu_s = 0.2$$, what is the maximum static friction force on the box?

Accepted Answer

$$f_{s,	ext{max}} = 0.2 	imes 1000 = 200\ \mathrm{N}$$

Question 6

Which of the following statements about friction is correct?

Accepted Answer

Kinetic friction is always less than or equal to static friction.

Question 7

A box of mass 10 kg is at rest on a horizontal surface. What is the normal force acting on the box?

Accepted Answer

$$F_N = 10 	imes 9.8 = 98\ \mathrm{N}$$

Question 8

A 20 kg box is sliding along a horizontal surface with a coefficient of kinetic friction $$\mu_k = 0.2. If $$the box is pulled horizontally with a force of 150 N, what is the net force acting on the box?

Accepted Answer

$$F_{	ext{net}} = 150 - (0.2 	imes 20 	imes 9.8) = 110.8\ \mathrm{N}$$

Question 9

Which equation correctly represents Newton's Second Law of Motion?

Accepted Answer

$$F = m a$$

Question 10

A box is at rest on a horizontal surface. If a force is applied but the box does not move, which force is balancing the applied force?

Accepted Answer

Static friction

Question 11

A 5 kg object is suspended by a rope. What is the tension in the rope if the object is at rest?

Accepted Answer

$$T = 5 	imes 9.8 = 49\ \mathrm{N}$$

Question 12

In a pulley system, two blocks are suspended on a line. Block 1 has mass $$m_1$$ and block 2 has mass $$m_2$$ ($$m_2$$ \u003e $$m_1). $$Which statement is correct about the tension in the string?

Accepted Answer

The tension is the same throughout the string.

Question 13

A boy is sliding down a frictionless plane. Which force is responsible for his acceleration?

Accepted Answer

The component of gravity parallel to the plane

Question 14

Which of the following is the correct formula for the maximum static friction force?

Accepted Answer

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

Question 15

A force is applied to a box on a rough surface, but the box does not move. What does this indicate about the applied force?

Accepted Answer

It is less than or equal to the maximum static friction force.

Question 16

A 10 kg box is pulled across a horizontal surface with a force of 60 N. If the coefficient of kinetic friction is $$\mu_k = 0.3$$, what is the acceleration of the box?

Accepted Answer

$$a = \frac{60 - (0.3 	imes 10 	imes 9.8)}{10} = 3.06\ \mathrm{m/s^2$}$$

Dynamics: Forces, Friction, Free Body Diagrams, and Tension

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