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Re: Why is it "static friction?"



Static friction just means there is not motion across the interface, so
if the block is not moving relative to the table the force is static.

It seems to me that you question is the same as:

why does the ball keep rolling across the floor when nothing is pushing
it? or
if the earth is attracted to the sun by the gravitational force, why
doesn't it fall into the sun and burn up?

I think the arguement has to be that since the block is accelerating,
there must be a net force on it, and the only candidate is the contact
force by the table on it. We call such a shear force friction, and
since the block is not moving relative to the table, the force is
static friction.

That is an example of what I call Newtonian logic, that only makes
sense if you buy into the Newtonian hypothesis. So its a
self-consistent explanation, which might be valid.

cheers,

joe

On Thu, 13 Nov 2003,
Ludwik Kowalski wrote:

I have no doubt that friction has something to do
with turning. Without friction a car would continue
to move along the straight line, no matter how its
wheels are turned. But this alone does not justify
the statement that m*v^2/r=mu*m*g.

On Thursday, November 13, 2003, at 04:02 PM, Ludwik Kowalski wrote:

Why do we say that the m*v^2/r (acting on a car) is
the force of static friction?

I think of static friction as a "responding" force. For
example, a crate pulled to the right (by a rope) will
experience a responding force (from the floor) directed
to the left. If the pulling force (cause) is small then the
responding force (effect) is also small. The net force
acting on the crate remains zero when the causing
force increases (up to a limit). Kinetic frictional force
acts in the direction opposite to motion, static frictional
force acts in the direction opposite to the direction of
a causing force.

Referring to the centripetal force acting on a turning
car a textbook states "the force in the radial direction
acting on the car is the force of static friction directed
toward the center of the circular path." If the static
friction force is the effect then where is the causing
force? I am not convinced that the centripetal force,
acting on a turning car, is the static friction force (as
it was introduced in the first paragraph above).
Ludwik Kowalski


Joseph J. Bellina, Jr. 574-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556