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Third Law of Motion

The third law of motion is not so much a law about motion as it is a rule of thumb about pairs of forces. It is usually stated thus:
 

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Third Law of Motion
This is certainly poetic, but what does it mean? More clear (and less poetic) is the following:
If object 1 exerts a force   on object 2, then object 2 exerts a force  on object 1
Which is equal in magnitude, opposite in direction, and of the same type (gravity, friction, etc.):
 
 

 

Example

 The Sun and the Earth exert a force of gravity on each other. Draw a force diagram.

 

Solution

See Figure 3-8.

 

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Figure 3-8

 

FES = gravitational force of the Earth on the Sun,

FSE = gravitational force of the Sun on the Earth.
 

 

Example

Two spacecraft push off from each other. Draw a force diagram.

 

Solution

See Figure 3-9.

 

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Figure 3-9

 

F12 = contact force of craft 1 on craft 2,

F21 = contact force of craft 2 on craft 1.
 

 

Example

A basketball player jumps up. While he is in the air, he pushes the basketball horizontally. Draw all the forces while he is pushing the ball. Ignore the tiny gravitational force between the player and the ball.

 

Solution

See Figure 3-10.

 

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Figure 3-10

 

Fpb = contact force of player on ball,
Fbp = contact force of ball on player,
FEp = gravitational force of Earth on player,
FpE = gravitational force of player on Earth,
FEb = gravitational force of Earth on ball,
FbE = gravitational force of ball on Earth.
 

Notice that the magnitude of the force of the player on the ball is the same as that of the ball on the player. But the player moves hardly at all, while the ball springs toward another player. Why is the basketball affected more than the player? (Hint: Look at equation 1.)
 





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