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Re: Pushing a car uphill



It has been our experience that it is more difficult to
start pushing a stalled car on a level surface than it is
to keep it rolling once the car has started moving.
Why is this true??

Could this be simply a delta-momentum argument? From speed 0 to some
low speed v, you have to change the momentum of the car by m*v.

But once at the [low] speed v, air friction is negligible and the
decrease in momentum due to rolling losses (primarily the result of
having to continuously deform the tires?) is -relatively- small
compared to the force required to get to the original m*v, hence the
need for less force to keep it rolling...

I'm neglecting the delta-t required to equate force to delta-p,
having assumed it does not play a significant role in comparing the
"start from zero" and "start from v" cases. Any experiments designed
to get to the bottom of the question would have to take delta-t into
account as a controlled variable.


Stefan Jeglinski