# interference fit

```In message <199801241903.LAA13062@gonzo.wolfenet.com> Orin Eman writes:

> > All OK too. Just to make terms easier, acceleration, in either direction
> > is still acceleration, or increasing velocity over time. Deceleration is
> > slowing down, reducing velocity over time, direction of motion up or
> > down is irrelevant.
>
> Unfortunately, in this case, we have to take direction into consideration.

Well, we should really drift off into calculus.

However, to assist visualisation, think of a bungee jump.

A person jumps, and is accelerated towards the ground by gravity.  The
rope comes tight, and starts to stretch - exerting an increasing force
that starts to exert a decellerating effect.

There is an instant at which the jumper is stationary, before being
dragged back up by the rope.  The force being exerted on the jumper
is _greatest_ at this very instant, when the rope is at maximum
stretch.  Since the mass of the jumper doesn't change (excepting
inadvertent egestion) the acceleration (product of force acting on
a mass) is also greatest at this instant.

It's exactly the same in a car engine - the acceleration is greatest
when motion drops to zero.  Practically, it means that this is the
instant when the crank/connecting rod/piston are under the greatest
strain - any tolerances in the big and little end bearings will
cause the piston to move even further into the danger area.

Actually, piston movement in a car is sinusoidal.  I failed A-level
maths thirty years ago, but ISTR that the result of differentiating
a sine curve is to shift it 90 degrees?

--
Phil Payne
phil@isham-research.demon.co.uk
Phone: +44 385302803  Fax: +44 1536723021

```