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Re: That jerk again



Hi Folks,

I could no longer resist contributing to the discussion of Leigh's
experiments in stopping his car. It seems to me that it is useful to
consider the motion from the frame of reference of my car, which is moving
down the hill at a constant non zero velocity (relative to the road).
Let's agree that his car comes to rest, relative to the road, at time T.
In the discussion that follows the motion of Leigh's car is
described from the frame of my car.

Before time T the Leigh's car is moving up the hill with decreasing speed,
so it's acceleration is directed down the hill. It follows that there is
a net force in the same direction. Following time T it moves up the hill
at constant speed and zero acceleration. The car is now in equilibrium
with the frictional force acting as the equilibrant to the normal and
weight forces. It is at time T that this force is suddenly applied and
Leigh and his car are jolted (jerked) by its sudden application.

Gene Mosca