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Lesson: Newton's Third Law of Motion

In this lesson, we will learn how to apply Newton's third law of motion to analyze systems of forces.

Sample Question Videos

04:30

Worksheet • 4 Questions • 1 Video

Q1:

Which of the following statements most correctly describes Newton’s third law of motion?

  • AWhen a force is applied to an object, the object exerts an equal-sized force in the opposite direction to the applied force.
  • BWhen a force is applied to an object, the object exerts an equal-sized force in the direction of the applied force.
  • CWhen a force is applied to an object, the object exerts a force in the opposite direction to the applied force that is proportional to the mass of the object that the force is applied to.
  • DWhen a force is applied to an object, the object exerts a force in the opposite direction to the applied force that is proportional to the mass of the object that applies the force.

Q2:

A stone of mass 686 g is thrown vertically upward from Earth with a force of 45 N.

What magnitude force does the gravitational field of Earth exert on the stone?

What magnitude force does the gravitational field of the stone exert on Earth?

Which of the following statements most correctly describes the motion of Earth due to the gravitational field of the stone?

  • AThe magnitude of the acceleration of Earth due to the stone’s gravitational field is equal to the force applied by the gravitational field of the stone divided by Earth’s mass.
  • BEarth is not accelerated at all by the gravitational field of the stone.
  • CThe magnitude of the acceleration of Earth due to the stone’s gravitational field is equal to the mass of the stone divided by Earth’s mass.
  • DThe magnitude of the acceleration of Earth due to the stone’s gravitational field is equal to the acceleration of the stone but in the opposite direction.

What magnitude force does the stone exert on the person who throws it?

Q3:

A rock of mass 5 kg is dropped onto Earth and decelerates from a speed of 1 m/s to rest in a time of 0.1 s.

What magnitude force does the deceleration of the rock exert on Earth?

What magnitude force does the collision with Earth exert on the rock?

Which of the following statements most correctly describes the motion of Earth due to the collision with the rock?

  • AThe magnitude of the acceleration of Earth due to the collision is equal to the force applied in the collision divided by Earth’s mass.
  • BEarth is not accelerated at all by the collision with the rock.
  • CThe magnitude of the acceleration of Earth due to the collision is equal to the mass of the rock divided by Earth’s mass.
  • DThe magnitude of the acceleration of Earth due to the collision is equal to the acceleration of the rock but in the opposite direction.

Q4:

A block has a mass of 500 g. The block is part of a system of objects. One end of the block is attached to a string and the other end of the string is attached to a ball by passing over a pulley, as shown in the diagram. The ball is inside a box that also contains a spring. A hole in a wall of the box allows the string to enter the box, pass through the gap in the center of the spring, and connect to the ball. All the objects in the system are at rest.

How much force does the block apply to the string?

How much force does the string apply to the ball?

How much force does the ball apply to the spring?

How much force does the spring apply to the ball?

How much force does the ball apply to the string?

How much force does the string apply to the block?
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