Newton’s Third Law
Contact Interactions
Action­at­a­Distance Interactions
• According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. • When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. • There are two forces resulting from this interaction ­ a force on the chair and a force on your body. • These two forces are called action and reaction forces and are the subject of Newton's third law of motion. Formally stated, Newton's third law is:
Whenever one object exerts a force on a second object , the second object exerts a force that is equal in magnitude and opposite in direction tto the first object.
*The action and reaction forces are acting on different objects.
*In every interaction, there is a pair of forces (action force and reaction force) acting on the two interacting objects. 1
Examples of action­reaction force pairs:
• A hand pushes on a book with a force of 10N [left]. The book pushes on the hand with a force of 10N [right]. • Water pushes a fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). • What are the action and reaction forces in the pictures below:
tractor on rope
rope on tractor
Check Your Understanding at the website below:
http://www.physicsclassroom.com/Class/newtlaws/u2l4a.cfm
Homework: p.118 # 1-2, p.119 # 2-5
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Newton’s 3rd Law Problems
Ex. A 40 kg toboggan with a child on it is connected with a rope to a 30kg toboggan with another child on it. They are being pulled east with a force of 140N. What are the action­ reaction pair of forces between the toboggans?
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Ex.6 p. 117 Two blocks at rest on a frictionless, horizontal surface. Block X has a mass of 6.0 kg and is attached by means of a light taut string to block Y that has a mass of 12 kg. A force of 36 N [E] is applied to block X. Calculate the force block Y exerts on block X and verify Newton's 3rd Law.
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