# Conrod force/acceleration

```In message <34CC6849.9380FB3@novagate.com> Sargent Schutt writes:

> Phil Payne wrote:
>
> > In message <34CBEB4D.B0BB2005@novagate.com> Sargent Schutt writes:
> >
> > > When there is no velocity, there is no
> > > acceleration. Acceleration can be zero while velocity is constant, but if
> > > velocity = 0, acceleration = 0; acceleration is nothing more than the rate
> > > of change in velocity. At TDC velocity is zero, and therefore acceleration
> > > is zero, too. Yet the engine is most definitely running.
> >
> > No - almost completely wrong.
> >
> > And, if velocity is constant, acceleration _MUST_ be zero.
> >
> > Think again of the bungee jumper - his BDC (zero velocity) is the moment of
> > _maximum_ acceleration.  The rope exerts its greatest force when it is fully
> > stretched (Hook's Law).
>
> BDC would be the moment of zero acceleration. In the moment *immediately*
> preceeding and following would be his maximum acceleration. There is an instant
> in time when velocity, and therefore acceleration are zero.
^^^^^^^^^

In the case of the velocity of the piston - it changes direction.  Expressed in
any units, the sign changes and there is therefore a point at which the graph
crosses the X axis.  At this point, velocity _is_ zero.

There is no such change (at TDC/BDC, anyway) in acceleration - the sign doesn't
change as slowing in one direction gives way to speeding up in the other, and
thus the graph doesn't cross the X axis and there's no instantaneous zero.

--
Phil Payne
Committee Member, UK Audi [ur-]quattro Owners Club

```