Newton's First Law of Motion - Balanced Forces.
A force is a push or a pull. It is measured in Newtons (N).
If all the forces which act on
an object
along the same straight line
are equal and opposite,
then when you subtract one side
from the other
the resultant force
is zero and the forces are called balanced.
The picture below is called a free-body
force diagram.
It shows only the object
and the forces acting on it.
The arrow shows the direction
of the force and the
length of the arrow
shows the size of the force
(see vectors).
The cube in the picture has two forces pulling in opposite directions.
The resultant force is 5 - 5 Newtons = 0 N. The
forces are balanced.
Newton's First Law says that
If the forces on an object are balanced
then it will carry on as it is.
This means that
(i) if the object is
not moving then it will continue to stay
still.
(ii) if the object is
already moving
then it will continue to move with
a constant
velocity.
Part (ii) goes against common
sense.
We know that in the "real world"
things slow down and stop
if there is nothing to keep them
going.
Newton understood that there is a
force
which makes things slow down and
stop. It is called friction.
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