Newton’s Third Law

Chapter 6

Forces and Interactions• Forces result from the interactions

between objects. Examples are:– gravitational attraction between the earth and

objects on its surface.

– a push against a wall results in the wall pushing back.

– the weight of a book and the support or normal force a table exerts on the book.

– a hammer exerts a force on a nail and the nail also exerts a force on the hammer.

Newton’s Third Law of Motion• Whenever an object exerts a force on a second object,

the second object exerts an equal and opposite force on the first.

OR• For every action of a force, there is an equal and

opposite reaction force.

• Forces always act in pairs, constituting an interaction:: “It “It takes two to tango!”takes two to tango!”

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Actions and Reactions

More action/reaction

• Action: Object A exerts a force on object B• Reaction: Object B exerts a force on object A• Incorrect for situation: Apples falls to ground

– Action: Earth pulls Apple– Reaction: Apple falls to the ground

• Correct– Action: Earth pulls Apple– Reaction: Apple pulls Earth

Action/Reaction on different masses• If the force on the apple and the Earth

is equal, why does the apple move but the Earth does not move?

• Acceleration depends on force and mass. The force are the same but the masses are not.

• The Earth has a large mass, so it has a very small acceleration.

• The apple has a small mass and a large acceleration

Accelerations due to Action- Reaction Forces

• Although action and reaction forces are equal in magnitude, the resulting accelerations depend on the masses (inertias) of the interacting objects.

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FF

A puzzle, the Horse and Cart • The horse pulls the

cart with a force of 20 N to the right.

• According to Newton, the cart pulls the horse with a force -20 N to the left.

• So how can they start moving, or accelerate?

Action/Reaction and Net Force• Action/Reaction forces act on

different objects so they do not cancel

• Net forces act on the same object so if they are equal and opposite they can cancel to produce a net force of zero. The normal force and the weight both act on the apple.

• In the horse and cart problem, consider each part separately, and don’t forget that there are other forces acting.

Newton’s Third Law Questions1) While driving, Anna observed a bug striking the windshield of her car.

Obviously, a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield? Explain.

2) A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is ... a) greater than the acceleration of the bullet. b) smaller than the acceleration of the bullet. c) the same size as the acceleration of the bullet.

3) In the top picture, a physics student is pulling upon a rope which is attached to a wall. In the bottom picture, the physics student is pulling upon a rope which is held by the Strongman. In each case, the force scale reads 500 Newtons. The students pulls…a) with more force when the rope is attached to the wall. b) with more force when the rope is attached to the Strongman. c)the same force in each case.

More Newton’s Third Law Questions1) For the following situations, list action and

reaction forces.a) A bat hits a ballb) Bowling ball pushes pin rightwardsc) A satellite orbits the Earth

2) Its is possible for a boxer to hit a wall with a force of 200 N. Is it also possible for the same boxer to hit a tissue with that same amount of force?

3) With what force does an apple weighing 1 N pull the Earth? Explain.

More review……

Arnold Strongman and Suzie Small pull on opposite ends of a rope in a tug of war. The greater force exerted on the rope is by

1. Arnold.2. Suzie.3. Neither. The force is the same.

1. 100 N,2. 200 N, or3. Zero?

Does the scale read:

Sources

• Conceptual Physics by Paul Hewitt• www.physicsclassroom.com• http://rigel.physics.unr.edu/faculty/phaneuf/c

lassinfo/index100.html