To achieve motion with constant velocity we can blow air between the
mating surfaces of the car and track thus almost removing the
frictional forces on the cart. To peform the measurement I push the
cart and activate the timing system simultaneously. I obtain the times
of passage of the four reference points. These numbers I can write into
a table
But it is even more useful to plot them in a position versus time plot.
If we had more photo-gates or a more elaborate way of figuring the
position at all times we could get more points on the curve. In this
case our four points already indicate that the position of the cart along
the track increases in proportion to the elapsed time.
The most important message I would like to get through to you today is that the position versus time curve is a complete description of the motion of a point-like object. From it you can if you wish derive additional diagnostics about the motion as we shall do in the following. However none of these diagnostics is as complete a description of the motion as the x(t) curve itself.