Q: A box with a weight of 100 N is in motion. It is then pulled by a 30 N horizontal force. If the friction force is 20 N, what is the net force acting on the box?

$$F_{\text{net}} = 30\ \mathrm{N} - 20\ \mathrm{N} = 10\ \mathrm{N}$$

Q: A block of mass $$m$$ slides down a ramp that makes an angle $$\theta$$ with the horizontal. The coefficient of friction between the block and the ramp is $$\mu. $$What is the acceleration $$a of $$the block?

$$a = g(\sin \theta - \mu \cos \theta)$$

Q: A 1200 kg car accelerates from rest to $$25\ \mathrm{m/s} in 10 $$seconds. What is the average net force exerted on the car?

$$F = m a = 1200 \times \frac{25}{10} = 3000\ \mathrm{N}$$

Q: A block is pulled across a horizontal surface with a force $$F at an $$angle $$\phi$$ above the horizontal. If the block's mass is $$m$$ and the friction force is $$f$$, which equation gives the net force in the horizontal direction?

$$F_{\text{net}} = F \cos \phi - f$$

Question 1

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 2

A 3 kg mass is suspended from a spring and stretches it by 0.1 m. What is the spring constant $$k$$? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$k = \frac{mg}{x} = \frac{(3)(9.8)}{0.1} = 294\ \mathrm{N/m}$$

Question 3

A box with a weight of 100 N is in motion. It is then pulled by a 30 N horizontal force. If the friction force is 20 N, what is the net force acting on the box?

Accepted Answer

$$F_{	ext{net}} = 30\ \mathrm{N} - 20\ \mathrm{N} = 10\ \mathrm{N}$$

Question 4

Which of the following is an example of Newton's Third Law?

Accepted Answer

Normal force and weight

Question 5

A block of mass $$m is $$accelerated across a rough surface by a force $$F at an $$angle $$\phi$$ above the horizontal. The frictional force is $$f. $$What is the horizontal acceleration $$a_x of $$the block?

Accepted Answer

$$a_x = \frac{F \cos \phi + f}{m}$$

Question 6

A block of mass $$m$$ slides down a ramp that makes an angle $$	heta$$ with the horizontal. The coefficient of friction between the block and the ramp is $$\mu. $$What is the acceleration $$a of $$the block?

Accepted Answer

$$a = g(\sin 	heta - \mu \cos 	heta)$$

Question 7

Using Newton's Law of Gravitation, derive the acceleration due to Earth's gravity $$g at $$the surface. Which equation is correct?

Accepted Answer

$$g = \frac{GM_E}{R_E^2$}$$

Question 8

Given the mass of the Earth $$M_E = 5.97 	imes 10$$^{24}$$\ \mathrm{kg}$$ and the radius of the Earth $$R_E = 6.37 	imes 10^6$\ \mathrm{m}$$, what is the value of $$g at $$the surface? ($$G = 6.67 	imes 10$$^{-11}$$\ \mathrm{N\,m^2$/kg^2$$}$)

Accepted Answer

$$g = 9.8\ \mathrm{m/s^2$}$$

Question 9

What is the orbital velocity $$v of $$the Earth around the Sun, given $$G = 6.67 	imes 10$$^{-11}$$\ \mathrm{N\,m^2$/kg^2$$}$$, M_S = 1.98$$ 	imes $$10^{30}$$\ \mathrm{kg}$$, and r = 1.49$$ 	imes $$10^{11}$$\ \mathrm{m}$?

Accepted Answer

$$v = \sqrt{\frac{GM_S}{r}}$$

Question 10

A 1200 kg car accelerates from rest to $$25\ \mathrm{m/s} in 10 $$seconds. What is the average net force exerted on the car?

Accepted Answer

$$F = m a = 1200 	imes \frac{25}{10} = 3000\ \mathrm{N}$$

Question 11

A block is pulled across a horizontal surface with a force $$F at an $$angle $$\phi$$ above the horizontal. If the block's mass is $$m$$ and the friction force is $$f$$, which equation gives the net force in the horizontal direction?

Accepted Answer

$$F_{	ext{net}} = F \cos \phi - f$$

Question 12

A 2 kg block slides down a ramp inclined at $$30^\circ$$ with a coefficient of friction $$\mu = 0.2. $$What is the acceleration of the block? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$a = 9.8(\sin 30^\circ - 0.2 \cos 30^\circ) = 3.0\ \mathrm{m/s^2$}$$

Question 13

Which of the following statements about friction is correct?

Accepted Answer

Friction always acts opposite to the direction of motion.

Physics 210 Exam 2 Practice – Newton’s Laws, Forces, and Gravitation

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