Q: A 5 kg box is pulled across a horizontal surface with a force of $$F = 20\ \mathrm{N} at an $$angle of $$30^\circ$$ above the horizontal. If the coefficient of kinetic friction is $$\mu_k = 0.2$$, what is the magnitude of the friction force?

$$F_f = \mu_k m g = 0.2 \times 5 \times 9.8 = 9.8\ \mathrm{N}$$

Q: A 3 kg object is moving in a circle of radius 2 m at a constant speed of 4 m/s. What is the magnitude of the net force acting on the object?

$$F = m v^2$ / r = 3 \times 16 / 2 = 24\ \mathrm{N}$$

Q: Which of the following is the correct expression for the vector sum (resultant) of two forces $$\vec{F}_1$$ and $$\vec{F}_2$$ acting on an object?

$$\vec{F}_{\text{net}} = \vec{F}_1 + \vec{F}_2$$

Question 1

Which of the following best describes the normal force acting on an object resting on a horizontal surface?

Accepted Answer

It acts perpendicular to the surface and supports the object against gravity.

Question 2

A 5 kg box is pulled across a horizontal surface with a force of $$F = 20\ \mathrm{N} at an $$angle of $$30^\circ$$ above the horizontal. If the coefficient of kinetic friction is $$\mu_k = 0.2$$, what is the magnitude of the friction force?

Accepted Answer

$$F_f = \mu_k m g = 0.2 	imes 5 	imes 9.8 = 9.8\ \mathrm{N}$$

Question 3

According to Newton's First Law, which of the following statements is true?

Accepted Answer

An object at rest will remain at rest unless acted on by a net external force.

Question 4

A car of mass $$m is $$moving at constant velocity on a straight road. Which of the following must be true about the forces acting on the car?

Accepted Answer

The net force on the car is zero.

Question 5

A block slides down a rough incline of angle $$	heta$$ with respect to the horizontal. Which of the following correctly gives the component of the gravitational force parallel to the incline?

Accepted Answer

$$mg \sin 	heta$$

Question 6

A 10 kg mass is suspended by a rope. What is the tension in the rope if the mass is at rest?

Accepted Answer

$$T = 98\ \mathrm{N}$$

Question 7

Which of the following is a correct statement of Newton's Second Law?

Accepted Answer

$$\sum \vec{F} = m \vec{a}$$

Question 8

A 2 kg object is acted on by a net force of $$6\ \mathrm{N}. $$What is its acceleration?

Accepted Answer

$$a = 3\ \mathrm{m/s^2$}$$

Question 9

Which of the following is NOT a property of a force?

Accepted Answer

A force always acts at a distance.

Question 10

A block is pulled by two forces: $$F_1 = 10$$\ \mathrm{N}$$ to the right and F_2$ = 6\ \mathrm{N} to $$the left. What is the net force on the block?

Accepted Answer

$$4\ \mathrm{N} to $$the right

Question 11

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

Accepted Answer

For every action, there is an equal and opposite reaction acting on different objects.

Question 12

A 3 kg object is moving in a circle of radius 2 m at a constant speed of 4 m/s. What is the magnitude of the net force acting on the object?

Accepted Answer

$$F = m v^2$ / r = 3 	imes 16 / 2 = 24\ \mathrm{N}$$

Question 13

Which of the following is the correct free-body diagram for a block sliding down a rough incline?

Accepted Answer

Weight, normal force perpendicular to the surface, and friction force up the incline.

Question 14

A 50 kg astronaut pushes on a 100 kg satellite in space. If the astronaut moves away at 2 m/s, what is the velocity of the satellite?

Accepted Answer

$$v = -1\ \mathrm{m/s}$$

Question 15

Which of the following is the correct expression for the vector sum (resultant) of two forces $$\vec{F}_1$$ and $$\vec{F}_2$$ acting on an object?

Accepted Answer

$$\vec{F}_{	ext{net}} = \vec{F}_1 + \vec{F}_2$$

Question 16

A block is pulled by a rope at an angle $$	heta$$ above the horizontal. Which trigonometric function gives the vertical component of the force?

Accepted Answer

$$F \sin 	heta$$

Question 17

A car of mass $$m is $$decelerating due to a friction force $$F_f. $$What is the acceleration of the car?

Accepted Answer

$$a = -F_f / m$$

Question 18

Which of the following is a correct step when drawing a free-body diagram (FBD)?

Accepted Answer

Draw only the forces acting on the object of interest.

Question 19

A block of mass $$m is $$sliding down a frictionless incline of angle $$	heta. $$What is its acceleration?

Accepted Answer

$$a = g \sin 	heta$$

Newton's Laws of Motion: Forces and Free-Body Diagrams

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