Q: A 4 kg mass and a 6 kg mass are both hung from the same string over a pulley. If the system is released, what is the acceleration of the system? (Assume $$g = 10 \ \mathrm{m/s^2$}$$ and ignore friction.)

$$a = \frac{(6-4) \times 10}{6+4} = 2 \ \mathrm{m/s^2$}$$

Question 1

According to Newton's Third Law, when a person pushes against a wall, the wall pushes back with an equal and opposite force. Which of the following best describes the action-reaction pair in this scenario?

Accepted Answer

The person exerts a force on the wall; the wall exerts an equal and opposite force on the person.

Question 2

A girl and a boy are sitting on nearly frictionless carts and pull on a rope between them. If the girl has less mass than the boy, what happens when they pull?

Accepted Answer

Both move toward each other, but the girl accelerates more than the boy.

Question 3

Which of the following statements about Newton's Third Law pairs is correct?

Accepted Answer

The two forces in a Newton's Third Law pair always act on different objects.

Question 4

A child jumps upward on a trampoline. While the child is accelerating upward, what is the action-reaction force pair?

Accepted Answer

The child pushes down on the trampoline; the trampoline pushes up on the child.

Question 5

A hammer drives a nail into a wooden board. According to Newton's Third Law, which statement is true?

Accepted Answer

The force the hammer exerts on the nail is equal in magnitude and opposite in direction to the force the nail exerts on the hammer.

Question 6

A 4 kg mass and a 6 kg mass are both hung from the same string over a pulley. If the system is released, what is the acceleration of the system? (Assume $$g = 10 \ \mathrm{m/s^2$}$$ and ignore friction.)

Accepted Answer

$$a = \frac{(6-4) 	imes 10}{6+4} = 2 \ \mathrm{m/s^2$}$$

Question 7

In a tug-of-war between The Hulk and Barry Leroy, both pull on the rope with equal and opposite forces. If The Hulk wins, what is the best explanation based on Newton's Laws?

Accepted Answer

The Hulk is able to exert a greater frictional force on the ground, allowing him to accelerate Barry Leroy.

Question 8

Which of the following is an example of an internal force within a system?

Accepted Answer

The force between two blocks connected by a rope within the system.

Question 9

A car accelerates forward due to friction between its tires and the road. What is the action-reaction pair involved in this scenario?

Accepted Answer

The tires push backward on the road; the road pushes forward on the tires.

Question 10

If a block is pulled to the right with a force $$F$$ and experiences a frictional force $$f to $$the left, what is the net force on the block?

Accepted Answer

$$F_{\mathrm{net}} = F - f$$

Question 11

A student steps forward and begins to walk. According to Newton's Third Law, what force propels the student forward?

Accepted Answer

The floor pushes forward on the student as the student pushes backward on the floor.

Question 12

Which of the following best describes the role of pulleys in a system of masses and strings?

Accepted Answer

Pulleys change the direction of forces without changing their magnitude (ideal case).

Question 13

A 10 kg block is pulled to the right with a force of 50 N and experiences a frictional force of 20 N to the left. What is the acceleration of the block?

Accepted Answer

$$a = \frac{50 - 20}{10} = 3 \ \mathrm{m/s^2$}$$

Question 14

Which of the following is the correct word-equation for Newton's Second Law as it applies to a system of objects?

Accepted Answer

Sum of external forces equals mass of system times acceleration of system.

Question 15

If the mass on one side of a two-mass pulley system is increased, what happens to the acceleration of the system (assuming all else is constant)?

Accepted Answer

The acceleration increases because the net external force increases.

Question 16

Which of the following is NOT a correct statement about Newton's Third Law force pairs?

Accepted Answer

The action and reaction forces can be different types of forces.

Question 17

A person pulls a sled across snow with a rope at an angle. Which force is responsible for the sled's forward acceleration?

Accepted Answer

The horizontal component of the tension in the rope.

Question 18

If the net external force on a system is zero, what can be said about the acceleration of the system?

Accepted Answer

The acceleration of the system is zero.

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