Question 1

A 10 kg box rests on a horizontal surface. The coefficient of static friction between the box and the surface is $$\mu_s = 0.5. $$What is the maximum horizontal force that can be applied to the box before it starts to move?

Accepted Answer

$$50\ \mathrm{N}$$

Question 2

A block is pulled across a horizontal surface at constant velocity. If the block has a weight of $$200\ \mathrm{N}$$ and the coefficient of kinetic friction is $$\mu_k = 0.3$$, what is the magnitude of the friction force acting on the block?

Accepted Answer

$$60\ \mathrm{N}$$

Question 3

A 500 N crate is at rest on a level floor. If a horizontal force of $$50\ \mathrm{N} is $$applied and the coefficient of static friction is $$\mu_s = 0.46$$, what is the friction force acting on the crate?

Accepted Answer

$$50\ \mathrm{N}$$

Question 4

A sled loaded with students (total weight $$w) $$slides down a snow-covered hill with coefficient of kinetic friction $$\mu_k at a $$constant velocity. What is the angle $$	heta of $$the hill in terms of $$\mu_k$$?

Accepted Answer

$$	heta = \arctan(\mu_k)$$

Question 5

Two boxes, A (20 kg) and B (5 kg), are in contact on a frictionless surface. A horizontal force of $$250\ \mathrm{N} is $$applied to box A. What is the magnitude of the force that box A exerts on box B?

Accepted Answer

$$192\ \mathrm{N}$$

Question 6

A block of mass $$m is $$pulled by a rope at an angle $$	heta$$ above the horizontal. The coefficient of kinetic friction is $$\mu_k. $$Which equation gives the normal force $$n on $$the block?

Accepted Answer

$$n = mg - T\sin	heta$$

Question 7

A 4.00 kg crate and a 6.00 kg crate are connected by a rope on a frictionless surface. If a person pulls the 6.00 kg crate with a force $$F so $$that both accelerate at $$2.50\ \mathrm{m/s^2$}$$, what is the tension $$T in $$the rope between the crates?

Accepted Answer

$$15.0\ \mathrm{N}$$

Question 8

A block is pulled to the right by a string and remains stationary. Which force balances the tension in the string?

Accepted Answer

The static friction force $$f_s$$

Question 9

Which of the following statements about tension in a massless, frictionless pulley system is correct?

Accepted Answer

The tension is the same on both sides of the pulley.

Question 10

A block is sliding at constant velocity across a horizontal surface. Which of the following is true about the forces acting on the block?

Accepted Answer

The pulling force equals the kinetic friction force.

Question 11

A block of mass $$m is $$connected by a light string over a frictionless pulley to a hanging mass $$M. If$$ $$M \u003e m$$, what is the acceleration $$a of $$the system?

Accepted Answer

$$a = \frac{(M - m)g}{M + m}$$

Question 12

Which of the following best explains why friction occurs between two surfaces?

Accepted Answer

Friction arises from molecular interactions at points where the surfaces touch.

Question 13

A block is pulled with a force $$T at an $$angle $$	heta$$ above the horizontal. If the block moves at constant velocity, which equation must be satisfied?

Accepted Answer

$$T\cos	heta = \mu_k n$$

Question 14

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

Accepted Answer

The coefficient of kinetic friction $$\mu_k is $$usually less than the coefficient of static friction $$\mu_s.$$

Question 15

A 500 N crate requires a 230 N horizontal force to start moving and a 200 N force to keep moving at constant velocity. What are the coefficients of static and kinetic friction?

Accepted Answer

$$\mu_s = 0.46,\ \mu_k = 0.40$$

Question 16

Which of the following is NOT a correct statement about tension in a rope?

Accepted Answer

Tension is always equal to the weight of the object attached.

Question 17

If a block is at rest and no horizontal force is applied, what is the value of the static friction force?

Accepted Answer

$$0$$

Question 18

Which of the following best describes the direction of the friction force?

Accepted Answer

It always opposes the direction of motion or impending motion.

Force Applications II: Friction, Interacting Objects, and Tension

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