# Re: Conrod force/acceleration

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From: Sargent Schutt <sargent@novagate.com>

>> If both acceleration and velocity are 0 at the same time, it is a
>> clear indication that the engine isn't running at all.

>Come again? Is there not a point in time where acceleration and velocity
>are zero? I certainly think so. 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.

Hello Sarge.  I am afraid, you're not quite correct with this assumption
that 0 velocity = 0 acceleration.  That is analagous to saying that 0
distance = 0 velocity which is also not true.

Think of an object thrown up in the air.  The only significant force on that
object is the force of gravity.  Any text book will show this to be a
downward (surprise, surprise) accelleration of 9.81m/s.  The object will
slow down, stop and then fall back to the ground again.  At all times, even
at the highest point (at which it is stopped) gravity is acting on it and
one of Newtons laws claims that force = mass x accelleration.  At the
beginning, this acceleration is in the opposite sense to the velocity, and
therefore slows it down.  Later, the acceleration is in the same direction
as the velocity and causes the velocity to increase as the object returns to
the ground.