So we start with problem number 1, which is 2.A.4.
It deals with spring gun and pucks on a frictionless
horizontal table.
And all this comes down to Newton's first
law, F equals ma.
And so I'll write that in glorious detail, one of the
key equations of 801.
F equals ma.
It's a vector equation.
a, the acceleration is in the same direction at any moment
in time that the force acts.
F in terms of magnitude, we express in terms of Newtons.
m, which is a scalar in kilograms.
And a, which is a vector, the magnitude of that is meters
per second squared.
If we take an example, if the magnitude of F-- and I'll put
vertical bars here to indicate that we're dealing with a
magnitude--
is 5 Newtons, if the mass were 2 kilograms, then the
magnitude of the acceleration were 2 and 1/2 meters per
second squared.
If there is no force, if the net force is 0, then the net
acceleration is 0.
If the net force is not 0, the net acceleration is not 0.
Now, be careful.
If the net force is 0, it does not mean necessarily--
it doesn't mean at all in general that the
object isn't moving.
No acceleration simply means that the
velocity is a constant.
It doesn't change in magnitude and it
doesn't change in direction.
So don't make the mistake that you think if that force is 0
and therefore, the acceleration, that the object
necessarily has to stand still.
That is not at all true.