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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) Its 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

How does looking at the question from an energy standpoint change things?
The only choices given involve forces as long as a level road can be assumed.
I see no other cause for the forward acceleration?

I'm late in adding a comment, but how can I resist? At the AAPT
meeting in Philly, van Heuvelen discussed this exact problem. (A car
accelerates forward on a level road. There is air resistance but
neglect the rolling resistance resulting from deformation of the tire
& road and friction in the wheel bearings.) He clearly had a circle
around the car indicating the choice of system and an arrow on his
diagram labeled "work of the road on the car." I put up my hand at
the end and asked him about it and he quietly said something to the
effect of "maybe that wasn't a very good choice of system."

I thought his answer missed the entire point, but I don't like to
make a scene so I let it pass then. But in this forum I would make
the following observations:

1. I can choose the system any way I like. Different choices lead to
different insights about the forces, work, momentum & energy
transfer, etc. I have discovered that certain textbooks insist that
systems must be chosen in only certain ways. In particular, some
books baldly state that one must never choose the system boundary to
be the "open" interface between two objects across which friction
acts. My response to this is: You are missing an opportunity to gain
additional, valuable insights. Who are you to tell me I cannot choose
my system other ways?

2. According to the pseudowork-kinetic-energy theorem, things are
clear. The net external force F on the car is static friction forward
minus air drag backward. (If you want to include rolling friction, as
Herb seems to want, toss that in as another backward term. Of course,
there is only one overall friction, but there is no harm in
artificially splitting it into pieces for discussion purposes.) The
net center-of-mass displacement dx is forward in some time interval
dt. Therefore, we have Fdx = d(mv^2/2). Simple and clear to students.
If you additionally assume F is constant, this agrees with an
analysis using Newton's second law plus the equations of kinematics
for constant acceleration.

3. Introducing thermodynamics does not help. The problem as stated is
a mechanics problem. Noting that the work done by static friction is
zero merely confuses students (and van Heuvelen in this case, oops).
Discussing how the internal energy of the gasoline becomes kinetic
energy of the car (and "thermal energy" of everything) is all well
and nice but not germane and certainly not easy to apportion out in
detail.

Carl (just tooting the "pseudowork" horn again)
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu http://physics.usna.edu/physics/faculty/mungan/