Re: Car acceleration

[Herbert H Gottlieb <herbgottlieb@JUNO.COM>]

On Fri, 01 Feb 2002 16:11:20 -0700 Gary Turner <turner@MORNINGSIDE.EDU>
writes:
> I came across this question in an FCI-type set of questions:
>
> A car accelerates forward from a stop sign.  Air resistance slows
> the motion.  What force causes the car to accelerate forward?
> A) It's weight
> B) The force of the engine on the tires
> C) The friction on the car from the road
> D) The upward force exerted on the car by the road
> E) The force of the driver on the accelerator pedal

I'm surprised at all of the incorrect answers that have
already been given by listmembers to this question.  In
my (not-so-humble) opinion, only choice A could be
correct.

A. We are not told if the car engine is operating
or if the road is level.  If the car accelerates
forward on a down-sloping road it must be due
to the component of the weight in the forward
direction.

B. The engine of a car never applies a force
directly to the tires.

c. Friction from the road would be in the reverse direction
and could only decelerate the car (which we assume is already
initally stopped at the stop sign).

D. An upward force from the road could result in a
negative acceleration . But the question did not specify
or imply a deceleration, or a negative acceleration.

E. The accelerator pedal is not directly connected to the
wheels.  If we reason that accelerator is indirectly connected
to the motion of a car, then we could also assume that
the breakfast of the driver and hundreds of other indirect
causes are reasonable .... and the question really becomes silly.

Herb Gottlieb from New York City
(Where cars do not always stop at stop signs when the police
are not around)