Q: Two boxes, A and B, are sliding to the right on a frictionless surface and are being slowed by a hand pushing on box A. Box A has a larger mass than box B. Considering only the horizontal forces, which of the following is true?

$$F_{\text{on H}} = F_{\text{on B}} \u003e F_{A \text{ on B}} = F_{B \text{ on A}}$$

Question 1

According to Newton's third law, which of the following statements is always true for an action/reaction pair of forces?

Accepted Answer

They act on different objects and are equal in magnitude but opposite in direction.

Question 2

Two blocks, A and B, are in contact and are pushed together on a frictionless surface. Block A has mass $$m_A$$ and block B has mass $$m_B. If a $$force $$F is $$applied to block A, what is the acceleration $$a of $$the system?

Accepted Answer

$$a = \dfrac{F}{m_A + m_B}$$

Question 3

When analyzing two objects in contact, which of the following steps is NOT necessary according to the general strategy outlined in the lecture?

Accepted Answer

Draw a single free-body diagram for both objects together.

Question 4

A 5.0 kg block A is pushed with a 3.0 N force against a 10 kg block B on a frictionless surface. What is the force that block A exerts on block B?

Accepted Answer

$$2.1\ \mathrm{N}$$

Question 5

Two boxes, A and B, are sliding to the right on a frictionless surface and are being slowed by a hand pushing on box A. Box A has a larger mass than box B. Considering only the horizontal forces, which of the following is true?

Accepted Answer

$$F_{	ext{on H}} = F_{	ext{on B}} \u003e F_{A 	ext{ on B}} = F_{B 	ext{ on A}}$$

Question 6

Which of the following pairs of forces is an action/reaction pair according to Newton's third law?

Accepted Answer

The friction force acting on A and the friction force acting on B

Question 7

A massless rope connects two objects and passes over a massless, frictionless pulley. If a force $$F$$ pulls on one end of the rope, what is the tension $$T in $$the rope at any point?

Accepted Answer

$$T = F$$

Question 8

A 22 kg wooden box is pulled at constant speed with a rope that makes an angle of $$25^\circ$$ with the horizontal. If the coefficient of kinetic friction is $$\mu_k$$, what is the correct expression for the tension $$T in $$the rope?

Accepted Answer

$$T = \dfrac{\mu_k m g}{\cos 25^\circ}$$

Question 9

Three 50-kg blocks are at rest, hanging vertically from ropes. How does the tension in rope 2 compare to the tension in rope 1?

Accepted Answer

Tension in rope 2 is equal to the tension in rope 1.

Question 10

A block of mass $$M is $$accelerated across a frictionless table by a falling mass $$m$$ connected by a massless string over a massless, frictionless pulley. What is the tension $$T in $$the string compared to $$mg$$?

Accepted Answer

$$T \u003c mg$$

Question 11

A 100 kg stagehand and a 30 kg stage set are connected by a massless rope over a frictionless pulley. If the stagehand lets go and is pulled upward while the set falls, what is the acceleration $$a of $$the system?

Accepted Answer

$$a = \dfrac{(m_h - m_s)g}{m_h + m_s}$$

Question 12

Which of the following is a correct statement about the tension in a massless rope passing over a massless, frictionless pulley?

Accepted Answer

The tension is the same at all points in the rope.

Question 13

A block is at rest on a horizontal surface. Which force is responsible for your sensation of weight when you stand on a scale?

Accepted Answer

The normal force exerted by the scale

Question 14

In a system where two objects are connected by a rope and move together, which of the following is always true?

Accepted Answer

The objects have the same acceleration.

Question 15

A 10 kg block is pulled by a rope with a force of 50 N at an angle of $$30^\circ$$ above the horizontal on a frictionless surface. What is the horizontal acceleration of the block?

Accepted Answer

$$a = \dfrac{50 \cos 30^\circ}{10}$$

Question 16

If a massless rope is used to pull a 5 kg block across a frictionless surface with a constant force of 20 N, what is the tension in the rope?

Accepted Answer

$$T = 20\ \mathrm{N}$$

Question 17

A 10 kg block is suspended by a massless rope. What is the tension in the rope if the block is at rest?

Accepted Answer

$$T = 98\ \mathrm{N}$$

Question 18

A 3 kg block and a 5 kg block are connected by a string and pulled to the right on a frictionless surface by a force of 20 N. What is the acceleration of the system?

Accepted Answer

$$a = \dfrac{20}{8}$$

Question 19

Which of the following best describes the effect of a massless, frictionless pulley on the tension in a rope passing over it?

Accepted Answer

The tension remains unchanged on both sides of the pulley.

L20: Interacting Objects, Contact Forces, and Applications of Newton's Third Law

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