Q: A box of mass $$5\ \mathrm{kg}$$ rests on a horizontal surface. If the coefficient of friction between the box and the surface is $$0.2$$, what is the magnitude of the frictional force acting on the box when a $$10\ \mathrm{N}$$ horizontal force is applied but the box does not move?

$$f = \mu n = 0.2 \times (5 \times 9.8) = 0.2 \times 49 = 9.8\ \mathrm{N}$$

Q: A $$70\ \mathrm{kg}$$ astronaut weighs less on the Moon than on Earth. If the acceleration due to gravity on the Moon is $$1.6\ \mathrm{m/s^2$}$$, what is the astronaut's weight on the Moon?

$$w = mg = 70 \times 1.6 = 112\ \mathrm{N}$$

Q: A $$100\ \mathrm{kg}$$ block is pulled by a $$500\ \mathrm{N}$$ force at $$30^\circ$$ above the horizontal. What is the normal force on the block? (Assume $$g = 9.8\ \mathrm{m/s^2$}$$ and ignore friction.)

$$N = mg - 500 \sin 30^\circ = 100 \times 9.8 - 500 \times 0.5 = 980 - 250 = 730\ \mathrm{N}$$

Question 1

Which of the following statements best describes a force in physics?

Accepted Answer

A force is a vector quantity that represents a push or pull, with both magnitude and direction.

Question 2

A box of mass $$5\ \mathrm{kg}$$ rests on a horizontal surface. If the coefficient of friction between the box and the surface is $$0.2$$, what is the magnitude of the frictional force acting on the box when a $$10\ \mathrm{N}$$ horizontal force is applied but the box does not move?

Accepted Answer

$$f = \mu n = 0.2 	imes (5 	imes 9.8) = 0.2 	imes 49 = 9.8\ \mathrm{N}$$

Question 3

A $$10\ \mathrm{N}$$ force is applied to a box at an angle of $$30^\circ$$ above the horizontal. What is the horizontal component of this force?

Accepted Answer

$$10 \cos 30^\circ = 8.66\ \mathrm{N}$$

Question 4

Which of the following is an example of a long-range force?

Accepted Answer

Weight (gravitational force)

Question 5

A hockey puck slides on a frictionless surface and is acted on by two horizontal forces, $$\vec{F}_1$$ and $$\vec{F}_2$$, such that $$\vec{F}_1 + \vec{F}_2 = 0. $$What happens to the puck?

Accepted Answer

It moves with constant velocity.

Question 6

According to Newton's second law, what is the acceleration of a $$2\ \mathrm{kg}$$ object when a net force of $$8\ \mathrm{N} is $$applied?

Accepted Answer

$$a = \frac{8}{2} = 4\ \mathrm{m/s^2$}$$

Question 7

A $$70\ \mathrm{kg}$$ astronaut weighs less on the Moon than on Earth. If the acceleration due to gravity on the Moon is $$1.6\ \mathrm{m/s^2$}$$, what is the astronaut's weight on the Moon?

Accepted Answer

$$w = mg = 70 	imes 1.6 = 112\ \mathrm{N}$$

Question 8

A block is pulled by a rope with tension $$T at an $$angle $$	heta$$ above the horizontal. Which equation gives the vertical component of the tension?

Accepted Answer

$$T \sin 	heta$$

Question 9

A $$100\ \mathrm{kg}$$ block is pulled by a $$500\ \mathrm{N}$$ force at $$30^\circ$$ above the horizontal. What is the normal force on the block? (Assume $$g = 9.8\ \mathrm{m/s^2$}$$ and ignore friction.)

Accepted Answer

$$N = mg - 500 \sin 30^\circ = 100 	imes 9.8 - 500 	imes 0.5 = 980 - 250 = 730\ \mathrm{N}$$

Question 10

A runner pushes against the starting block at an angle, generating a force with both horizontal and vertical components. Which statement is correct?

Accepted Answer

The horizontal component of the force accelerates the runner forward.

Question 11

A $$1\ \mathrm{kg}$$ box is acted on by a net force of $$5\ \mathrm{N}$$, resulting in an acceleration $$a_1 = 5$$\ \mathrm{$$m/s^2$$}$$. The same force is applied to a second box, producing an acceleration a_2$ = 1\ \mathrm{m/s^2$}. $$What is the mass of the second box?

Accepted Answer

$$m_2 = 5$$\ \mathrm{kg}$$

Question 12

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 13

A basketball player jumps by pushing down on the floor. What force causes the player to accelerate upward?

Accepted Answer

The normal force from the floor

Question 14

A puck moves at constant speed in a circle, attached to a rope. What is the direction of the net force acting on the puck?

Accepted Answer

Toward the center of the circle

Question 15

A $$2\ \mathrm{kg}$$ object is acted on by a net force of $$6\ \mathrm{N}$$ for $$3\ \mathrm{s}. If $$the object starts from rest, what is its final velocity?

Accepted Answer

$$v = \frac{6}{2} 	imes 3 = 3 	imes 3 = 9\ \mathrm{m/s}$$

Question 16

Which of the following is NOT a contact force?

Accepted Answer

Gravitational force (weight)

Question 17

A $$5\ \mathrm{kg}$$ object is suspended from a rope. What is the tension in the rope if the object is at rest? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

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

Question 18

A car of mass $$1000\ \mathrm{kg}$$ accelerates from rest under a net force of $$2000\ \mathrm{N}. $$What is its acceleration?

Accepted Answer

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

Newton's Laws of Motion: Forces and Dynamics

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