For every action, there is a reaction.

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For every action, there is a reaction.
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For every force, there is an equal and opposite
force.
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We can NOT be talking about a BALANCING force.
Why not?
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We can NOT be talking about a BALANCING force.
Why not?Because things do in fact accelerate
sometimes!
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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.
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Reaction forces are easy to nameJust reverse
the direction and reverse the rolesA table
pushes up on a book.The book pushes down on
the table.
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The only tricky one is weight. But if you
remember that weight is just the force of the
Earth pulling down on youThe Earth pulls down
on the student.The student pulls up on the
Earth.But which force is bigger?
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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.
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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!
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Then, one last puzzle If the forces are equal,
why dont they balance?In other words, where is
the flaw in this argumentWhen 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.
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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
ballForce of ball on player?
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They are acting on different objects!Action-reac
tion pairs never balance each other because they
do NOT act on the same object!