Static Friction
Holding in Place

Objects on an incline will often stay put.
There must be a force that holds the object in place.

Static friction is from the contact of resting objects.

• Force holds up to a certain point
• Force is based on the type of contact (rough, smooth)
• Maximum force is proportional to the pressing force of the
object (normal force)
Inequality

The approximate formula for static friction is:
F fr  m s FN
ms is the coefficient of static friction

This is an inequality.
• The force of static friction is generally less than the
coefficient times the normal force
Starting Motion

F fr  mg sin q
FN
mg sin q

If Ffr < msFN = msmg cosq,
then the block will hold.
At equality the block just
begins to move.
F fr  m s FN
q
mg cos q
Fg  mg
mg sin q  m s mg cos q
m s  tan q
Measuring Friction: Pulley

Use a pulley and vary the
weight to see when
movement begins.
• Forces balance on mass 1
F fr (max)  m s m1 g
Frope
m1
FT
• Tension is equal on the rope
FT
Frope
• Forces balance on mass 2
m2
Fg  m2 g
Coefficient of Friction: Pulley

The pulley determines the
coefficient of friction through
the ratio of masses.
m s m1 g  m2 g
m s  m2 / m1
F fr (max)  m s m1 g m1
m2
Fg  m2 g
Measuring Friction: Incline

Use an incline and vary the
angle to see when
movement begins.
• One component of gravity
balances the normal force
• The other component of
gravity balances friction
F fr (max)  m s mg cosq
FN
mg sin q
q
mg cos q
Fg  mg
Coefficient of Friction: Incline

The incline determines the
coefficient of friction through
the tangent of the angle.
mg sin q  m s mg cosq
m s  tan q
F fr (max)  m s mg cosq
mg sin q
q
Normal Force and Friction

Static friction depends on both the normal force and
on the coefficient of friction.

To reduce friction requires reducing one of those
factors.
• Reduce normal force by lightening the load
• Reduce normal force by adding additional upward force
• Add a lubricant to reduce the coefficient of friction
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