Q: Once a block starts sliding down an inclined plane, what is its acceleration $$a in $$terms of $$g$$, $$\theta$$, and the coefficient of kinetic friction $$\mu_k$$?

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

Question 1

A car is driving on a wet road and suddenly needs to stop to avoid hitting a deer. If the car's wheels lock and it skids, which type of friction determines the stopping distance, and how does the mass of the car affect the stopping distance?

Accepted Answer

Kinetic friction determines the stopping distance, and the stopping distance is independent of mass.

Question 2

Which equation correctly defines the friction coefficient $$\mu in $$terms of the friction force $$|f|$$ and the normal force $$N$$?

Accepted Answer

$$\mu = |f| / N$$

Question 3

A box of mass $$m is at $$rest on a horizontal surface. An applied force $$F is $$gradually increased. At what value of $$F$$ will the box just begin to slide, given the coefficient of static friction $$\mu_s$$?

Accepted Answer

$$F = \mu_s N$$

Question 4

Why do ABS brakes improve a car's ability to stop quickly compared to locked wheels?

Accepted Answer

ABS brakes allow the tires to maintain rolling contact, maximizing static friction ($$f_s \u003e f_k).$$

Question 5

A box is pushed against a vertical wall with a horizontal force $$F. $$What is the minimum force $$F$$ required to keep the box from sliding down, given the coefficient of static friction $$\mu_s$$ and the box's weight $$mg$$?

Accepted Answer

$$F = mg / \mu_s$$

Question 6

A block of mass $$m is $$placed on an inclined plane at angle $$	heta. At $$what angle will the block just begin to slide, given the coefficient of static friction $$\mu_s$$?

Accepted Answer

$$	heta = \arctan(\mu_s)$$

Question 7

Once a block starts sliding down an inclined plane, what is its acceleration $$a in $$terms of $$g$$, $$	heta$$, and the coefficient of kinetic friction $$\mu_k$$?

Accepted Answer

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

Question 8

Which statement best describes the difference between static and kinetic friction?

Accepted Answer

Static friction resists the initiation of motion, while kinetic friction resists motion once it has started.

Question 9

A person pulls a box of mass $$m$$ across a horizontal floor by applying a force $$F at an $$angle $$	heta$$ above the horizontal. What is the expression for the normal force $$N$$ acting on the box?

Accepted Answer

$$N = mg - F \sin	heta$$

Question 10

If a box is moving at constant velocity across a surface, what can be said about the net force acting on it and the relationship between the applied force and the kinetic friction force?

Accepted Answer

The net force is zero, and the applied force equals the kinetic friction force.

Question 11

Which of the following factors does NOT affect the value of the friction coefficient $$\mu$$ between two surfaces?

Accepted Answer

The gravitational acceleration $$g$$

Question 12

A truck and a car, both traveling at the same speed, lock their wheels and skid to a stop. Neglecting air resistance, which vehicle will have a longer stopping distance?

Accepted Answer

Both will have the same stopping distance because stopping distance does not depend on mass.

Question 13

Which of the following is an empirical property in the study of friction?

Accepted Answer

Friction coefficient $$\mu$$

Question 14

A box is at rest on a surface. An applied force $$F is $$less than the maximum static friction force. What is the value of the friction force acting on the box?

Accepted Answer

Equal to the applied force $$F$$

Question 15

When analyzing friction on an inclined plane, why is it useful to resolve forces into components parallel and perpendicular to the surface?

Accepted Answer

It simplifies the calculation of the normal force and friction force.

Question 16

Which of the following best describes the model for friction presented in introductory physics?

Accepted Answer

It is a simple model that works most of the time.

Question 17

A box is pulled across a horizontal surface with a force $$F at an $$angle $$	heta$$ above the horizontal. What is the minimum force $$F$$ required to move the box at constant velocity, given mass $$m$$, coefficient of kinetic friction $$\mu_k$$, and $$g$$?

Accepted Answer

$$F = \frac{\mu_k mg}{\cos	heta + \mu_k \sin	heta}$$

Question 18

If the coefficient of kinetic friction $$\mu_k$$ between a box and the floor is 0.3, and the normal force is 200 N, what is the magnitude of the kinetic friction force?

Accepted Answer

$$f_k = 60 	ext{ N}$$

Physics 211: Module 08 – Friction

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