From: Mark Lawton (markslawton@hotmail.com)
Date: Fri Nov 28 2003 - 12:45:10 PST
Date: Fri, 28 Nov 2003 12:45:10 -0800 Subject: more on sliding plates friction From: Mark Lawton <markslawton@hotmail.com> Message-ID: <BBECF3D6.4F56%markslawton@hotmail.com>
More on plates and friction...
When one pulls a tablecloth slowly the plates on it move due to friction.
Yes, in fact this is a restrictive force.
Think of it this way....
When you pull the tablecloth to the right, the friction is trying to hold
the tablecloth back (restrict it). It pushes to the left. That is, the
plates cause a force on the cloth to the left and by newton's third law the
cloth reacts by pushing the plates to the right.
This is the same reason your car moves to the right when you step on the
gas. The wheels rotate and push the road to the left (you can actually see
this if the road has loose dirt or gravel on it). The road pushes back and
makes the car move to the right.
-Mark Lawton
Portland, Oregon
>
>> 2) THE DEFINITION OF FRICTION I GIVE KIDS: Every
>> textbook defines friction as a resistive force in
>> general, or else specifically defines kinetic friction
>> as opposite in direction to velocity and static
>> friction as opposite to the sum of the other forces.
>> Yet, there are some cases that this doesn't extend
>> to...for example, a table cloth is dragged slowly and
>> the plates on top of it start to move with it. So,
>> it's really the friction that accelerates the plates.
>> So, in this case, friction isn't really resisting
>> motion or outside forces...not much of a "resistive
>> force" at all unless you want to talk about resisting
>> inertia, which is just awful. Is there a more general
>> definition that is simple enough for kids to
>> understand?
>>
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