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Static Friction Revisited
Renate Fiora
What is static friction?
• Static friction Is the force that keeps an
object “stuck” on a surface and prevents
its motion.
– The direction of static friction is opposite
the direction the object would move if there
were no friction.
• In other words, the force of friction points in
the direction needed to prevent motion.
Determining the magnitude is a
little weirder…
• Consider the situation where
someone is pushing on an unmoving box.
– If there is no velocity, then there is no
acceleration.
– No acceleration means no net force, so the force
of static friction must equal to the applied force.
• What is the person pushes harder but there’s
still no movement?
– Well, the force of static friction must still be equal
to the applied force.
So, the force of static friction increases as
the applied force increases up to some
maximum value above which the object
finally moves.
• What factors affect this maximum static friction?
1. The types of surfaces involved.
•
A smooth surface is more slippery (has less static
friction) than a rough surface.
2. The force pushing the two surfaces together.