A force is something that can
cause matter to start moving, stop moving, speed up or slow down. In
other words, if a force is applied to some piece of matter it will
change the way it is moving or not moving.
The problem with this is that it seems
to violate our common experience. We see moving objects slow down and
stop all of the time without any apparent force applied. To make
matters worse, there are lots of times that we know a force is being
applied and yet objects don’t start moving at all. To appreciate
forces, and to understand how they work, we need to imagine an
“ideal” world—that is a world where we can control all
forces.
As a simple example, think about a puck
on an air hockey table. This essentially eliminates friction, the
force that slows things down as they slide along the floor or a
table. Now, imagine the puck moving along the table. It glides, not
speeding up, not slowing down and not changing direction until some
force is applied, that is until something pushes it or pulls on it in
some way. This could be when it bangs into the wall, when it gets hit
by a paddle or even if you blow on it. Any of these things could stop
the puck, change its direction, slow it down, speed it up, or some
combination of these. What is important to recognize is that once the
puck is moving, force no longer needs to be applied to keep it
moving, but that to change the movement in any way (speed or
direction) a force must be applied.
Now imagine that both you and your
opponent are pushing on the puck at the same time. If you push in the
same direction, the puck will move faster and faster in that
direction. However, if you push in opposite directions, and if you
both push equally hard, the puck will not move. In this instance we
say that there are equal and opposite forces on the puck. Now, here’s
the weird part. If both of you were invisible, there would be no way
for someone looking at the puck, seeing it not moving, to tell
whether it was being pushed in opposite directions, or not being
pushed at all.
The upshot of this is that when
something is not moving, there are two possibilities. One, that there
is no force acting on it at all, and two, that there are equal and
opposite forces acting on it. In our very real world with friction,
gravity, etc. the second is much more common.
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