Q: A block slides down a rough incline of angle $$\theta$$ with respect to the horizontal. The coefficient of kinetic friction is $$\mu_k. $$Which equation correctly gives the acceleration $$a of $$the block?

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

Q: A block of mass $$m is $$being pulled up a rough incline at constant speed. The incline makes an angle $$\theta$$ with the horizontal, and the coefficient of kinetic friction is $$\mu_k. $$What is the magnitude of the pulling force $$F$$ parallel to the incline?

$$F = m g (\sin\theta + \mu_k \cos\theta)$$

Question 1

A 70 kg astronaut is standing on a scale inside an elevator that is accelerating upward at $$2 \ \mathrm{m/s^2$}. $$What is the apparent weight shown on the scale?

Accepted Answer

$$840 \ \mathrm{N}$$

Question 2

A box of mass $$m is at $$rest on a horizontal surface. The coefficient of static friction between the box and the surface is $$\mu_s. $$What is the maximum horizontal force that can be applied to the box before it starts to move?

Accepted Answer

$$\mu_s m g$$

Question 3

A car of mass $$1200 \ \mathrm{kg} is $$moving at a constant velocity on a flat road. If the coefficient of kinetic friction between the tires and the road is $$0.3$$, what is the magnitude of the frictional force acting on the car?

Accepted Answer

$$3600 \ \mathrm{N}$$

Question 4

A block slides down a rough incline of angle $$	heta$$ with respect to the horizontal. The coefficient of kinetic friction is $$\mu_k. $$Which equation correctly gives the acceleration $$a of $$the block?

Accepted Answer

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

Question 5

A skydiver of mass $$80 \ \mathrm{kg}$$ reaches terminal velocity during free fall. If the drag force is given by $$D = C_D \rho A v^2$ / 2$$ and at terminal velocity $$D = mg$$, which of the following is true?

Accepted Answer

$$C_D \rho A v^2$ / 2 = mg$$

Question 6

Which of the following statements about the normal force is correct?

Accepted Answer

The normal force is always perpendicular to the surface of contact.

Question 7

A 10 kg box is pulled across a horizontal floor with a constant velocity by a rope at an angle of $$30^\circ$$ above the horizontal. If the coefficient of kinetic friction is $$0.2$$, what is the magnitude of the frictional force?

Accepted Answer

$$20 \ \mathrm{N}$$

Question 8

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

Accepted Answer

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

Question 9

A person stands on a scale in an elevator that is accelerating downward at $$3 \ \mathrm{m/s^2$}. If $$the person's mass is $$60 \ \mathrm{kg}$$, what does the scale read?

Accepted Answer

$$420 \ \mathrm{N}$$

Question 10

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

Accepted Answer

Static friction prevents motion up to a maximum value; kinetic friction acts when the object is sliding.

Question 11

A 5 kg object is dropped from rest and falls through air. If the drag force is proportional to $$v^2$$, what happens to the acceleration as the object speeds up?

Accepted Answer

It decreases and approaches zero.

Question 12

A block is at rest on an incline. The angle of the incline is increased until the block just begins to slide. If the coefficient of static friction is $$\mu_s$$, what is the critical angle $$	heta_c at $$which sliding begins?

Accepted Answer

$$	heta_c = \arctan(\mu_s)$$

Question 13

Which of the following is a correct statement of Newton’s third law?

Accepted Answer

For every force, there is an equal and opposite force acting on a different object.

Question 14

A 2 kg block is pushed against a wall with a horizontal force of $$50 \ \mathrm{N}. $$The coefficient of static friction between the block and the wall is $$0.4. $$What is the maximum weight of the block that can be supported without slipping?

Accepted Answer

$$20 \ \mathrm{N}$$

Question 15

A baseball is thrown horizontally through air. Which of the following best describes the drag force acting on the ball at high Reynolds number?

Accepted Answer

It is proportional to the square of the velocity.

Question 16

A block of mass $$m is $$being pulled up a rough incline at constant speed. The incline makes an angle $$	heta$$ with the horizontal, and the coefficient of kinetic friction is $$\mu_k. $$What is the magnitude of the pulling force $$F$$ parallel to the incline?

Accepted Answer

$$F = m g (\sin	heta + \mu_k \cos	heta)$$

Question 17

Which of the following is NOT a correct step in drawing a free-body diagram?

Accepted Answer

Include forces acting on other objects.

Question 18

A 0.1 mg bacterium swims in water (low Reynolds number). If the drag force is proportional to velocity, what happens when the bacterium stops swimming?

Accepted Answer

It quickly comes to rest.

Applying Newton’s Laws: Equilibrium, Dynamics, Friction, Drag, and Interacting Objects

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