Q: A 10-kg box rests on a horizontal floor. If the coefficient of static friction between the box and the floor is 0.4, what is the maximum force that can be applied horizontally before the box starts to move? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

$$F_{\text{max}} = \mu_s m g = 0.4 \times 10 \times 9.8 = 39.2\ \mathrm{N}$$

Question 1

Which of the following best describes the direction of the frictional force acting on an object sliding across a horizontal surface?

Accepted Answer

Opposite to the direction of the object's motion.

Question 2

A 10-kg box rests on a horizontal floor. If the coefficient of static friction between the box and the floor is 0.4, what is the maximum force that can be applied horizontally before the box starts to move? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F_{	ext{max}} = \mu_s m g = 0.4 	imes 10 	imes 9.8 = 39.2\ \mathrm{N}$$

Question 3

Which of the following statements about static and kinetic friction is correct?

Accepted Answer

It takes more force to start moving an object than to keep it moving.

Question 4

A person pushes a refrigerator across a kitchen floor at a constant velocity. Which of the following is true about the forces involved?

Accepted Answer

The force of friction is equal and opposite to the person's push.

Question 5

Which of the following factors does NOT affect the magnitude of friction between two surfaces?

Accepted Answer

The speed of the object (for kinetic friction).

Question 6

A 5-kg iron ball and a 10-kg iron ball are dropped from rest in a vacuum. Which statement is correct?

Accepted Answer

Both balls have the same acceleration.

Question 7

If the mass of an object is halved, what happens to its weight on Earth?

Accepted Answer

It is halved.

Question 8

A skydiver jumps from a plane and accelerates downward. As her speed increases, what happens to the air resistance acting on her?

Accepted Answer

It increases.

Question 9

When a 20-N falling object encounters 5 N of air resistance, what is its acceleration? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$a \u003c g$$

Question 10

A 500-N person falls at terminal velocity. What is the magnitude of the air resistance acting on the person?

Accepted Answer

500 N.

Question 11

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

Accepted Answer

$$F = m a$$

Question 12

A cart is pushed along a track with a constant force. If the mass of the cart is halved, what happens to its acceleration?

Accepted Answer

It doubles.

Question 13

A heavy and a light person jump together with same-size parachutes from the same altitude. Who will reach the ground first?

Accepted Answer

The heavy person.

Question 14

Which of the following best describes terminal velocity?

Accepted Answer

The velocity at which the net force on a falling object is zero.

Question 15

A car of mass $$m is $$moving at velocity $$v$$ through air of density $$\rho$$ with a frontal area $$A. $$The drag force $$D$$ acting on the car is proportional to which of the following?

Accepted Answer

$$D \propto \rho A v^2$

Question 16

Which of the following is true about the mass and weight of an object?

Accepted Answer

Mass is constant, but weight varies with gravitational field strength.

Question 17

A 5-kg iron ball and a 10-kg iron ball are dropped together from rest. When the 5-kg ball reaches a speed of 10 m/s, what is the speed of the 10-kg ball?

Accepted Answer

10 m/s.

Question 18

If you push a box and it starts to move, which coefficient should you use to calculate the frictional force while it is sliding?

Accepted Answer

Coefficient of kinetic friction ($$\mu_k)$$

Question 19

A block is sliding at constant velocity across a horizontal surface. Which of the following statements is true?

Accepted Answer

The net force on the block is zero.

Chapter 4 newtons second law

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