pinhole Re: forces/friction

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From: Christopher Reese (creese@seq.org)
Date: Thu Oct 23 2003 - 11:23:40 PDT 

Message-Id: <sf97ba4e.097@smtp-gw.seq.org>
Date: Thu, 23 Oct 2003 11:23:40 -0700
From: "Christopher Reese" <creese@seq.org>
Subject: pinhole Re: forces/friction

The net force would be in the direction of friction (the only force
present in your example). Remind your students force causes
acceleration...the acceleration is in the opposite direction of
motion...causes the car or ball to slow. It won't ever move "backward"
(opposite direction) because friction can only oppose or impede motion
never cause it.

I hope this helps.

Christopher Reese
Science Teacher
Woodside High School
199 Churchill Ave
Woodside, CA 94062
www.seq.org/~creese
650-367-9750 x8660

>>> jbrotman52@hotmail.com 10/22/03 04:05PM >>>
hi again,
just as i think i'm starting to get this crazy world of physics, my
students
continue to ask me questions that challenge (and wear out!) my already

physics-challenged brain! i'd love some help.

so, when an object is moving at a constant velocity, there is not net
force
on that object. this means that if i am pushing something with a force
of
100N and it is travelling at a constant velocity, then the force of
friction
must be 100N the other way to balance that out. similarly, if i am
driving
a car at a constant velocity, and the engine is exerting a force of
100N,
then the force of friction balances that out with 100N the other way
(is
that oversimplified but right?). now, let's say, i suddenly stop the
car
mid-motion and turn off the engine. the car will slow down and
eventually
stop because of the force of friction, which is counteracting the car's

"desire" to keep moving because of its inertia, right?.

But how do we describe the forces on the car during its slow-down? My

student asked me if the only force on the car were friction in the
opposite
direction, why the car wouldn't move backwards. intuitively it makes
sense
that the car gradually slows down because of friction, but how do you
describe it in terms of the net force on the car? is there still the
force
of the car engine acting in the forward direction even when the car
shuts
off (and even so, if the forces were balanced before, it seems the net
force
would still be in the direction of the friction force)? is it the
ground
exerting a forward force on the car?

i guess a similar example would be rolling a ball and letting go. the
ball
will reach a constant velocity where the net force is zero, but what
other
force is at work as the ball is being slowed down by friction in the
opposite direction?

i'm sorry if that was long and confusing, but if anyone understood it
and
can shed some light, i'd really appreciate it!

thanks so much,
jennie

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