For every action, there is a reaction.
For every force, there is an equal and opposite force.
We can NOT be talking about a BALANCING force. Why not?
We can NOT be talking about a BALANCING force. Why not?
Because things do in fact accelerate sometimes!
Whenever one object exerts a force on a second object, the
second object ALWAYS exerts a force back on the first object.
We call the first force the “action” and we call the second force
its “reaction.
There is ALWAYS a reaction.
It is ALWAYS in the opposite direction.
It is ALWAYS exactly as big as the action.
Reaction forces are easy to name:
Just reverse the direction and reverse the roles…
“A table pushes up on a book.”
“The book pushes down on the table.”
The only tricky one is “weight”. But if you remember that
weight is just the force of the Earth pulling down on you…
“The Earth pulls down on the student.”
“The student pulls up on the Earth.”
But which force is bigger?
Sometimes, it is hard to believe that the forces are equal. For
example, when a car collides with a fly, when you name the
action and reaction forces…
…it is hard to accept that those two forces are equal in
magnitude.
But remember, the rule is that the FORCES are equal, not the
resulting accelerations! If the two forces are each the only
forces acting on their respective objects, then both objects
WILL accelerate. But the accelerations will be INVERSELY
PROPORTIONAL to the masses!
Then, one last puzzle: If the forces are equal, why don’t they
balance?
In other words, where is the flaw in this argument:
“When you kick a soccer ball, it can never accelerate because the
force your foot exerts on the ball is no bigger than the force the
ball exerts on your foot.”
Since soccer balls DO accelerate when you kick them, something
must be wrong with that argument.
The answer to the puzzle is revealed with free-body diagrams!
Make a free-body diagram for the soccer ball and another for the
soccer player.
What do you notice about the action-reaction pair: Force of player
on ball—Force of ball on player?
They are acting on different objects!
Action-reaction pairs never balance each other because they
do NOT act on the same object!