From: Paul Doherty (pauld@exploratorium.edu)
Date: Sun Oct 26 2003 - 19:02:40 PST
Message-Id: <l031107d5bbc23a131067@[192.168.112.78]> Date: Sun, 26 Oct 2003 19:02:40 -0800 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole Re: forces/friction
Hi Jennie
Friction for engine driven rolling wheels is more complicated than friction
for sliding bricks.
>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?.
The friction forces on a car moving at constant velocity are:
Wind resistance opposing the direction of motion and ground friction
pushing the car forward.
That's right. The ground pushes forward on the car because the car pushes
backward on the ground both of these are friction forces but we only care
about the one of them "On the Car"
Turn off the engine and the car doesn't push back on the ground anymore so
air resistance and other smaller internal friction forces which cause the
car wheels to push forward on the ground and so cause the ground to push
backward on the car bring it to rest.
When the car is at rest the fristion force of the ground on the car is zero.
So while the car is coasting forward the friction force is backward, as the
car slows this force diminishes to zero. So the car is not driven backward
by this force.
Paul D
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