Q: A 45 kg dresser rests on the floor. The coefficient of static friction between the dresser and the floor is $$\mu_s = 0.77. $$What is the maximum force that can be applied horizontally before the dresser starts to move?

$$F_{max} = 0.77 \times 45 \times 9.8 = 339.57\ \mathrm{N}$$

Q: A car at rest at a stop sign has a coefficient of static friction between its tires and the road of $$\mu_s = 0.95. $$What is the largest acceleration the car can have without the tires slipping?

$$a_{max} = \mu_s g = 0.95 \times 9.8 = 9.31\ \mathrm{m/s^2$}$$

Q: A car traveling at $$18.7\ \mathrm{m/s}$$ skids to a stop. If the coefficient of kinetic friction between the tires and the road is $$\mu_k = 0.72$$, what is the acceleration of the car while skidding?

$$a = -\mu_k g = -0.72 \times 9.8 = -7.056\ \mathrm{m/s^2$}$$

Q: A box is sliding on a surface with $$\mu_k = 0.4$$ and normal force $$F_N = 100\ \mathrm{N}. $$What is the kinetic friction force?

$$F_f = 0.4 \times 100 = 40\ \mathrm{N}$$

Question 1

A 45 kg dresser rests on the floor. The coefficient of static friction between the dresser and the floor is $$\mu_s = 0.77. $$What is the maximum force that can be applied horizontally before the dresser starts to move?

Accepted Answer

$$F_{max} = 0.77 	imes 45 	imes 9.8 = 339.57\ \mathrm{N}$$

Question 2

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

Accepted Answer

The surface area of contact

Question 3

A person pushes a refrigerator with a force of 150 N, and it does not move. What is the size of the friction force acting on the refrigerator?

Accepted Answer

150 N

Question 4

The graph of friction force versus applied force shows a linear increase up to a maximum value, then a sudden drop to a lower, constant value. What does the constant value represent?

Accepted Answer

The kinetic friction force

Question 5

A car at rest at a stop sign has a coefficient of static friction between its tires and the road of $$\mu_s = 0.95. $$What is the largest acceleration the car can have without the tires slipping?

Accepted Answer

$$a_{max} = \mu_s g = 0.95 	imes 9.8 = 9.31\ \mathrm{m/s^2$}$$

Question 6

A crate is in the back of a truck that accelerates from rest to $$21.6\ \mathrm{m/s} in$$ $$13.5\ \mathrm{s}. $$What is the minimum coefficient of static friction required to prevent the crate from sliding?

Accepted Answer

$$\mu_s = \frac{a}{g} = \frac{1.6}{9.8} = 0.163$$

Question 7

Which equation correctly expresses the relationship between friction force and normal force for kinetic friction?

Accepted Answer

$$F_f = \mu_k F_N$$

Question 8

A car traveling at $$18.7\ \mathrm{m/s}$$ skids to a stop. If the coefficient of kinetic friction between the tires and the road is $$\mu_k = 0.72$$, what is the acceleration of the car while skidding?

Accepted Answer

$$a = -\mu_k g = -0.72 	imes 9.8 = -7.056\ \mathrm{m/s^2$}$$

Question 9

If the applied force on an object exceeds the maximum static friction, what happens?

Accepted Answer

The object begins to move and kinetic friction takes over

Question 10

Which of the following best describes the role of the coefficient of friction?

Accepted Answer

It quantifies how easily two surfaces slide against each other

Question 11

A block is placed on a horizontal surface. If the normal force is doubled, what happens to the maximum static friction force?

Accepted Answer

It doubles

Question 12

A 325 N force pushes north on a dresser with a normal force of 441 N and $$\mu_s = 0.77. $$What will happen to the dresser?

Accepted Answer

It will not move because $$F_{applied} \u003c F_{max,static}$$

Question 13

Which of the following is true about kinetic friction compared to static friction for the same surfaces?

Accepted Answer

Kinetic friction is usually less than static friction

Question 14

A box is sliding on a surface with $$\mu_k = 0.4$$ and normal force $$F_N = 100\ \mathrm{N}. $$What is the kinetic friction force?

Accepted Answer

$$F_f = 0.4 	imes 100 = 40\ \mathrm{N}$$

Question 15

Which scenario would require a higher coefficient of static friction to prevent slipping?

Accepted Answer

A truck accelerating quickly with a crate in the back

Question 16

If the surface area of contact between two objects is increased, what happens to the friction force?

Accepted Answer

It remains unchanged

Question 17

A block is on an inclined plane. Which force is responsible for the block sliding down the plane?

Accepted Answer

The component of gravity parallel to the plane, $$mg \sin 	heta$$

Question 18

A person pushes a refrigerator with a force of 200 N, and it begins to accelerate. What is the size of the friction force acting on the refrigerator at that moment?

Accepted Answer

Less than 200 N

Question 19

Which of the following is a correct free-body diagram for a car skidding to a stop due to kinetic friction?

Accepted Answer

Forces: gravity downward, normal force upward, kinetic friction opposite to motion

Static and Kinetic Friction: Principles, Applications, and Problem Solving

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