Q: A 15.0 kg box A and a 5.0 kg box B are in contact on a frictionless horizontal surface. You push on box A with a horizontal force of 100 N. What is the acceleration of the system?

$$a = \frac{100\ \text{N}}{15.0\ \text{kg} + 5.0\ \text{kg}} = 5\ \text{m/s}^2$$

Q: For the scenario above, what is the force exerted by box A on box B?

$$F = m_B \cdot a = 5.0\ \text{kg} \cdot 5\ \text{m/s}^2 = 25\ \text{N}$$

Question 1

A 15.0 kg box A and a 5.0 kg box B are in contact on a frictionless horizontal surface. You push on box A with a horizontal force of 100 N. What is the acceleration of the system?

Accepted Answer

$$a = \frac{100\ 	ext{N}}{15.0\ 	ext{kg} + 5.0\ 	ext{kg}} = 5\ 	ext{m/s}^2$$

Question 2

For the scenario above, what is the force exerted by box A on box B?

Accepted Answer

$$F = m_B \cdot a = 5.0\ 	ext{kg} \cdot 5\ 	ext{m/s}^2 = 25\ 	ext{N}$$

Question 3

According to Newton’s Third Law, how does the force that box A exerts on box B compare to the force that box B exerts on box A?

Accepted Answer

Box A exerts as much force on box B as box B exerts on box A.

Question 4

A toboggan loaded with students slides down a frictionless hill at angle $$\alpha. $$What is the acceleration of the toboggan?

Accepted Answer

$$a = g \sin \alpha$$

Question 5

For the toboggan above, what is the magnitude of the normal force $$n$$ exerted by the hill?

Accepted Answer

$$n = mg \cos \alpha$$

Question 6

Two blocks are connected by a string over an ideal pulley. Block A slides on a frictionless incline at $$30^\circ$$, and block B hangs vertically. If $$m_A$$ and $$m_B$$ are their masses, what is the acceleration of the system?

Accepted Answer

$$a = \frac{m_B - m_A \sin 30^\circ}{m_A + m_B} g$$

Question 7

In the Atwood machine, two masses $$M = 0.6 kg $$and $$m = 0.4 kg $$are connected by a string over a frictionless pulley. What is the acceleration of mass $$M$$?

Accepted Answer

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

Question 8

A monkey and a bunch of bananas, each of mass 20 kg, hang from opposite ends of a rope over a pulley. If the monkey climbs up, what happens to the bananas?

Accepted Answer

The bananas move up.

Question 9

If the monkey releases her hold and both monkey and bananas are in free fall, what happens to the distance between them?

Accepted Answer

The distance remains constant.

Question 10

Which equation describes the kinetic friction force acting on a sliding object?

Accepted Answer

$$f_k = \mu_k n$$

Question 11

A 500-N crate requires a 230-N force to start moving and a 200-N force to keep moving at constant velocity. What is the coefficient of static friction?

Accepted Answer

$$\mu_s = \frac{230}{500} = 0.46$$

Question 12

For the crate above, what is the coefficient of kinetic friction?

Accepted Answer

$$\mu_k = \frac{200}{500} = 0.40$$

Question 13

A falling object reaches terminal speed when:

Accepted Answer

The drag force equals the weight of the object.

Question 14

For a skydiver of mass $$m$$ falling through air, the terminal speed $$v_t is $$given by:

Accepted Answer

$$v_t = \sqrt{\frac{mg}{D}}$$

Question 15

If a block is at rest and you apply a force less than the maximum static friction, what happens?

Accepted Answer

The block remains at rest.

Question 16

Which of the following best describes the relationship between friction and normal force?

Accepted Answer

Friction force is proportional to normal force.

Question 17

A block slides across a surface with kinetic friction. If the normal force is doubled, what happens to the kinetic friction force?

Accepted Answer

It is doubled.

Question 18

Which material pair has the highest coefficient of static friction according to the table?

Accepted Answer

Copper on cast iron

Question 19

A block is pulled across a level floor at constant velocity. Which statement is true about the forces acting on the block?

Accepted Answer

The pulling force equals the kinetic friction force.

Newton’s Laws of Motion and Applications: Forces, Friction, and Fluid Resistance

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