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

$$f_{s,\mathrm{max}} = 0.4 \times 98 = 39.2\ \mathrm{N}$$

Q: A 15 kg crate is at rest on a wooden floor. The coefficient of static friction between the crate and the floor is 0.5. What is the minimum horizontal force required to start moving the crate? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

$$F = 0.5 \times 147 = 73.5\ \mathrm{N}$$

Q: A 20 kg box is pulled across a horizontal floor with a constant velocity. If the coefficient of kinetic friction is 0.3, what is the magnitude of the frictional force? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

$$f_k = 0.3 \times 196 = 58.8\ \mathrm{N}$$

Q: A refrigerator (mass 80 kg) is pushed across a kitchen floor. The coefficient of kinetic friction between the refrigerator and the floor is 0.2. What is the minimum force required to keep the refrigerator moving at a constant velocity? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

$$F = 0.2 \times 784 = 156.8\ \mathrm{N}$$

Question 1

A person pushes a heavy couch across a carpeted floor. Which of the following best explains why it is harder to start moving the couch than to keep it moving at a constant speed?

Accepted Answer

The coefficient of static friction is greater than the coefficient of kinetic friction.

Question 2

A 10 kg box rests on a horizontal surface. If the coefficient of static friction between the box and the surface is 0.4, what is the maximum static friction force that can act on the box? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$f_{s,\mathrm{max}} = 0.4 	imes 98 = 39.2\ \mathrm{N}$$

Question 3

Which of the following changes will increase the frictional force between your hand and a table as you slide your hand across it?

Accepted Answer

Increasing the normal force by pressing harder.

Question 4

A 15 kg crate is at rest on a wooden floor. The coefficient of static friction between the crate and the floor is 0.5. What is the minimum horizontal force required to start moving the crate? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F = 0.5 	imes 147 = 73.5\ \mathrm{N}$$

Question 5

A box is pulled with a horizontal force of 30 N but does not move. If the coefficient of static friction is 0.4 and the box has a mass of 10 kg, what is the value of the static friction force acting on the box?

Accepted Answer

$$30\ \mathrm{N}$$

Question 6

Which of the following statements about static friction is correct?

Accepted Answer

Static friction can take any value up to its maximum, depending on the applied force.

Question 7

A 20 kg box is pulled across a horizontal floor with a constant velocity. If the coefficient of kinetic friction is 0.3, what is the magnitude of the frictional force? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$f_k = 0.3 	imes 196 = 58.8\ \mathrm{N}$$

Question 8

A student claims that increasing the surface area of contact between two objects will increase the frictional force. Which statement best evaluates this claim?

Accepted Answer

The claim is incorrect because friction depends on the normal force and the coefficient of friction, not the area of contact.

Question 9

A 25 kg box is pulled with a force of 120 N on a surface with $$\mu_s = 0.5$$ and $$\mu_k = 0.3. $$Will the box move? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

Yes, because $$120\ \mathrm{N} \u003e \mu_s n$$

Question 10

A refrigerator (mass 80 kg) is pushed across a kitchen floor. The coefficient of kinetic friction between the refrigerator and the floor is 0.2. What is the minimum force required to keep the refrigerator moving at a constant velocity? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F = 0.2 	imes 784 = 156.8\ \mathrm{N}$$

Question 11

A box is at rest on a horizontal surface. Which of the following best describes the relationship between the applied force and the static friction force before the box moves?

Accepted Answer

The static friction force matches the applied force up to its maximum value.

Question 12

A 10 kg box is pulled with a 50 N horizontal force. The coefficient of static friction is 0.4 and the coefficient of kinetic friction is 0.3. What is the acceleration of the box after it starts moving? (Use $$g = 10\ \mathrm{m/s^2$}$$ for simplicity)

Accepted Answer

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

Question 13

Which of the following is true about kinetic friction?

Accepted Answer

It is constant for a given normal force and coefficient of kinetic friction.

Question 14

A block is at rest on a horizontal surface. A horizontal force is gradually increased until the block just begins to move. Which equation correctly describes the maximum static friction force?

Accepted Answer

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

Question 15

A 12 kg box is on a horizontal surface. If a person pulls upward on the box with a rope while trying to slide it horizontally, what happens to the frictional force?

Accepted Answer

The frictional force decreases because the normal force decreases.

Question 16

A student pushes a box across a floor at constant speed. Which of the following statements is correct?

Accepted Answer

The applied force is equal to the kinetic friction force.

Question 17

Which of the following best describes the effect of mass on friction?

Accepted Answer

Mass affects the normal force, which in turn affects the frictional force.

EXAM #3 DAY 13 Friction: Modeling, Factors, and Problem Solving in Physics

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