Q: A 10 kg box rests on a horizontal surface. The coefficient of static friction between the box and the surface is 0.5. What is the minimum horizontal force required to start moving the box?

$$F = \mu_s n = 0.5 \times 10 \times 9.8 = 49\ \mathrm{N}$$

Q: A car rounds a flat curve of radius $$R at $$constant speed $$v. $$What is the magnitude of the net force required to keep the car moving in a circle?

$$F_{\text{net}} = \frac{m v^2$}{R}$$

Question 1

A car is parked on a flat road. Which of the following statements best describes the forces acting on the car according to Newton’s first law?

Accepted Answer

The net force on the car is zero, so $$\sum \vec{F} = 0$$ and the car is in equilibrium.

Question 2

A 5 kg box is pulled horizontally with a force of 20 N on a frictionless surface. What is the acceleration of the box?

Accepted Answer

$$a = \frac{F}{m} = \frac{20}{5} = 4\ \mathrm{m/s^2$}$$

Question 3

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

Accepted Answer

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

Question 4

A block is sliding at constant speed across a horizontal surface. Which statement is true about the forces acting on the block?

Accepted Answer

The kinetic friction force equals the applied force: $$f_k = \mu_k n$$

Question 5

A skydiver of mass $$m$$ falls through the air and eventually reaches terminal velocity. Which equation correctly describes the forces at terminal velocity?

Accepted Answer

$$D v_t^2$ = m g$$

Question 6

A ball attached to a string is whirled in a horizontal circle at constant speed. What is the direction of the net force acting on the ball?

Accepted Answer

Toward the center of the circle

Question 7

Which of the following is NOT a correct step in drawing a free-body diagram for a block sliding down an incline?

Accepted Answer

Including the vector $$m \vec{a} as a $$force acting on the block

Question 8

A 10 kg box rests on a horizontal surface. The coefficient of static friction between the box and the surface is 0.5. What is the minimum horizontal force required to start moving the box?

Accepted Answer

$$F = \mu_s n = 0.5 	imes 10 	imes 9.8 = 49\ \mathrm{N}$$

Question 9

A car rounds a flat curve of radius $$R at $$constant speed $$v. $$What is the magnitude of the net force required to keep the car moving in a circle?

Accepted Answer

$$F_{	ext{net}} = \frac{m v^2$}{R}$$

Question 10

Which of the following statements about friction is correct?

Accepted Answer

Kinetic friction acts when a body slides over a surface, and is given by $$f_k = \mu_k n.$$

Question 11

A block is at rest on a horizontal surface. A horizontal force is gradually increased until the block just begins to move. Which force is responsible for keeping the block at rest before it moves?

Accepted Answer

Static friction

Question 12

A 2 kg object is dropped from rest and falls under gravity with air resistance proportional to velocity ($$F_{	ext{drag}} = kv). $$What happens to the object’s velocity as time increases?

Accepted Answer

It approaches a constant terminal velocity.

Question 13

Which of the following is a correct statement of Newton’s second law in component form?

Accepted Answer

$$\sum F_x = m a_x$$

Question 14

A block is pulled at constant speed across a horizontal surface. If the coefficient of kinetic friction is doubled, what happens to the force required to keep the block moving at constant speed?

Accepted Answer

It doubles.

Question 15

Which of the following is NOT a fundamental force of nature?

Accepted Answer

Frictional force

Question 16

A ball attached to a string is moving in a horizontal circle. If the string suddenly breaks, what path will the ball follow?

Accepted Answer

It will move in a straight line tangent to the circle at the point of release.

Question 17

A windshield wiper squeaks on dry glass but not on wet glass. What is the best explanation for this phenomenon?

Accepted Answer

The coefficient of friction is higher when the glass is dry, causing stick-slip motion.

Applying Newton’s Laws: Equilibrium, Dynamics, Friction, and Circular Motion

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