From: Jennie Brotman (jbrotman52@hotmail.com)
Date: Wed Oct 22 2003 - 16:05:14 PDT
From: "Jennie Brotman" <jbrotman52@hotmail.com> Subject: Re: forces/friction Date: Wed, 22 Oct 2003 16:05:14 -0700 Message-ID: <BAY1-F55lJZ5sFmShG3000015a3@hotmail.com>
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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