Newton s third law of motion
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“For every action there is an equal and opposite reaction”. Newton’s Third Law of Motion:. The correct way to say it: . For every action force there is an equal and opposite reaction force . Forces always occur in PAIRS. An action force…. …and a r eaction force. Some examples :.

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Newton s third law of motion

“For every action there is an equal and opposite reaction”

Newton’s Third Law of Motion:

The correctway to say it:

For everyactionforcethere is an equal and oppositereactionforce.


Forces always occur in pairs
Forces always occur in PAIRS reaction”

An action force…

…and a reaction force


Some examples
Some examples reaction”:


See the pattern
See the pattern??? reaction”

A pushes on B, so B pushes on A

OR

A pulls on B, so B pulls on A


Question
Question: reaction”

If force pairs on two objects are always equal and opposite, how is it that sometimes only one of the objects moves????


Newton s third law of motion

For example: reaction”

Shooting an arrow?

Throwing a baseball?

Hitting a hockey puck?


Answer

Wait…… reaction”

What??

Answer:

The pair of forces acting on the objects has to be the same, but the REACTION of the objects to the forces does NOT have to be the same!


Newton s third law of motion

Example: a canon firing reaction”

According to Newton’s 3rd Law of Motion, both the canon ball and the cannon experience the SAME FORCE from the gunpowder.

So why does the cannon ball move so fast while the cannon only moves back a little?


Newton s third law of motion

Explanation 1 - The math: reaction”

The Cannon:

The Cannonball:

BIG mass:

SMALL mass

So according to Newton’s 2nd Law of Motion:

F

M

F

M

=

A

=

A

= SMALL acceleration!

= LARGE acceleration!


Newton s third law of motion

Explanation 2 – Inertia: reaction”

The cannon has much more INERTIA than the cannon ball.

Therefore the same force can move the cannon ball much more easily than it can move the cannon!