Q: A car is at rest at a stop sign. The coefficients of static and kinetic friction between the tires and the road are $$\mu_s = 0.95$$ and $$\mu_k = 0.72$$, respectively. What is the largest acceleration the car can achieve without the tires slipping?

$$a = 0.95 \times 9.8\ \mathrm{m/s^2$}$$

Q: A block slides down an inclined plane with angle $$\theta$$ and coefficient of kinetic friction $$\mu_k. $$What is the expression for the acceleration of the block?

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

Question 1

A 45 kg dresser rests on the floor. The coefficients of static and kinetic friction between the dresser and the floor are $$\mu_s = 0.77$$ and $$\mu_k = 0.59$$, respectively. If a 325 N force pushes north on the dresser, what will happen to the dresser?

Accepted Answer

The dresser will not move because the static friction force is greater than the applied force.

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 on a refrigerator with a force of 150 N, and it doesn’t move. What is the size of the friction force acting on the refrigerator?

Accepted Answer

150 N

Question 4

The friction force $$F_f is $$proportional to the normal force $$F_N. $$Which equation correctly expresses this relationship for kinetic friction?

Accepted Answer

$$F_f = \mu_k F_N$$

Question 5

A car is at rest at a stop sign. The coefficients of static and kinetic friction between the tires and the road are $$\mu_s = 0.95$$ and $$\mu_k = 0.72$$, respectively. What is the largest acceleration the car can achieve without the tires slipping?

Accepted Answer

$$a = 0.95 	imes 9.8\ \mathrm{m/s^2$}$$

Question 6

A block slides down an inclined plane with a constant velocity. The plane forms an angle $$	heta$$ with the horizontal. What is the formula for the coefficient of kinetic friction between the block and the surface?

Accepted Answer

$$\mu_k = 	an 	heta$$

Question 7

A crate is in the back of a parked pick-up truck. The truck accelerates from rest to $$21.6\ \mathrm{m/s} in$$ $$13.5\ \mathrm{s}. $$What is the minimum coefficient of static friction required so the crate does not slide within the truck?

Accepted Answer

$$\mu_s = 0.163$$

Question 8

Which of the following best describes the difference between static and kinetic friction?

Accepted Answer

Static friction acts to prevent motion up to a maximum value; kinetic friction acts when objects are sliding past each other.

Question 9

A block of wood is pushed into a wall with a force $$F at an $$angle $$38.0^\circ$$ below the horizontal. The coefficients of static and kinetic friction are $$\mu_s = 0.62$$ and $$\mu_k = 0.43$$, respectively. If the block slides up the wall with constant velocity, what is the magnitude of the friction force?

Accepted Answer

$$F_f = \mu_k F_N$$

Question 10

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 11

Which of the following is a correct statement about the graph of friction force versus applied force for a block on a surface?

Accepted Answer

The friction force increases linearly with applied force until the block starts moving, then remains approximately constant.

Question 12

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 $$\mu_k = 0.72$$, how long does it take for the car to stop? (Assume $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$2.05\ \mathrm{s}$$

Question 13

Which of the following surfaces has the lowest coefficient of kinetic friction according to the provided table?

Accepted Answer

Teflon on Teflon in air

Question 14

If a block slides down an inclined plane at constant velocity, what does this indicate about the net force acting on the block?

Accepted Answer

The net force is zero.

Question 15

A block of mass $$m is $$placed on a horizontal surface. If a horizontal force $$F is $$applied and the block does not move, which equation best describes the relationship between the applied force and the friction force?

Accepted Answer

$$F = F_f$$

Question 16

Which of the following is true about the coefficient of friction?

Accepted Answer

It depends on the materials in contact.

Question 17

A block is pushed up a wall with a force $$F at an $$angle. If the block moves at constant velocity, which forces must be balanced?

Accepted Answer

The sum of all forces in both the vertical and horizontal directions must be zero.

Question 18

A block slides down an inclined plane with angle $$	heta$$ and coefficient of kinetic friction $$\mu_k. $$What is the expression for the acceleration of the block?

Accepted Answer

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

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

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