Re: That jerk again!

[Herb Schulz <herbs@interaccess.com>]

Jim Riley said:

> Let us compare three situations.
>
> First, the car is at rest, not moving, has never been moving, and I apply
> the brakes.  Obviously I will feel nothing.
>
> Second, the car has been moving but under the influence of gravity the car
> and its contents have come instantaneously at rest.  Call this time t=0.   I
> apply the brakes at this instant.  Is there any physical difference between
> the objects involved at this instant and the previously mentioned condition?
> Answer-No.  Therefore I should feel nothing at this time either..
>
> Third the car has been moving under the influence of gravity, I do not apply
> the brakes, the car and contents at some instant have a zero velocity and
> then begin rolling backward, having at all time a constant acceleration, and
> no jerk.

Sorry,

There is a very large difference!  In the first case the velocity is zero
and the acceleration is also zero.  In the second and third cases the
velocity is zero (at that instant) but the acceleration is NOT zero.

This is one of the standard errors our student often make.  A static
situation and dynamic situation are NOT the same thing even though there
might be an instant where the velocities are both zero.

This is the same reason people don't understand why we have high tides on
opposite sides of the earth due to the moon's attraction of the water: they
think statically (so the water is all pulled to one side)!

Living in a non-inertial reference frame makes a difference.

In the case of the car, the position of the body of the car relative to the
chassis (i.e. the compression of the springs, etc.) could very well be
different in the static and dynamic situations Leigh talks about.

Good Luck,

Herb Schulz
(herbs@interaccess.com)