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Re: Car acceleration



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

Choice C could not be the "correct" answer to this problem.
In fact, none of the choices are correct by themselves
unless one makes many additional assumptions that are
not stated in the original problem.

Before the car can "accelerate forward from a stop sign"
the car is stopped and there is a static friction between
the car and the road. This force could not "cause the
car to accelerate forward".

It is a combination of many forces that causes the car to
accelerate forward. One of these is the rolling friction
between the car and the road. But the weight of the car
is also involved...as are the forces exerted by the
driver and by the engine.

Herb Gottlieb from New York City
(Where friction does not accelerate cars unless other
forces are also involved)


One could argue that E is the correct answer: The force of the foot
on the
pedal causes a chain of events with the result that the car
accelerates.

No, I am not just being facetious. The joker in this deck is the
word
"causes". This word as commonly used (and I don't remember it
being
redefined in any physics curriculum) does not preclude any number
of
intermediate agencies, time delays, etc.

A better wording might be "What force acts on the car to accelerate
it
forward?"
Chris

* * * * * * * * * * * * *

Christopher A. Horton, Ph.D.
4158 RR#3 (Hwy. 204)
Amherst, NS B4H 3Y1
CANADA
ChrisAHorton2@hotmail.com
(902) 447-2109

* * * * * * * * * * * * *

"Many discoveries are reserved for ages still to come, when memory
of us
will have been effaced. Our universe is a sorry little affair
unless it has
in it something for every age to investigate ... Nature does not
reveal her
mysteries once and for all."
- Seneca, "Natural Questions", first century, quoted by Carl Sagan
in
"Cosmos", p.xi.

* * * * * * * * * * *


----- Original Message -----
From: Gary Turner <turner@MORNINGSIDE.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, February 01, 2002 7:11 PM
Subject: Car acceleration


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

When I first read this, I thought I got the answer (which was the
'correct'
answer). That is below. Then I thought about it some more from an
energy
standpoint - now I'm not so sure. What does the group think?








Answer: C