From: Mark Lawton (markslawton@hotmail.com)
Date: Thu Oct 23 2003 - 22:14:22 PDT
Date: Thu, 23 Oct 2003 22:14:22 -0700 Subject: friction From: Mark Lawton <markslawton@hotmail.com> Message-ID: <BBBE033E.4AC9%markslawton@hotmail.com>
I don't quite see any problem with your initial thinking. Before the
friction force can make the car move backwards it slows the car down. Just
like before gravity can pull a baseball back to earth it slows down it's
upward velocity.
check out these diagrams...
Case I) car moving at constant velocity
-----------> car <-----------
A B
A = force of road on wheels which is a reaction to the wheels pushing on
road. (Of course, the wheels turn initially because of the engine.)
B = force of wind and other forms of friction
When moving at constant speed A = -B (same magnitude, different direction)
Case II) car slowing down because engine is off
car <-----------
B
A=0 B=force of wind and other forms of friction
car moving right but slowing down due to force B.
When the car finally comes to a standstill there is no more wind. Then
there is only (static) friction which will inhibit motion in any direction
until there is sufficient net force to overcome the static friction. In
other words the car will stay still because the engine is still off. When
you turn the engine on, it spins the wheels, which in turn push on the road,
which in turn pushes back on the car hard enough to overcome the static
friction.
> 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?
>
There is no other force slowing the ball down other than friction.
-Mark Lawton
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