1. Static Friction

2. 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 pointForce holds up to a certain point Force is based on the type of contact (rough, smooth)Force is based on the type of contact (rough, smooth) Maximum force is proportional to the pressing force of the object (normal force)Maximum force is proportional to the pressing force of the object (normal force)

3. Inequality  The approximate formula for static friction is:  s is the coefficient of static friction  This is an inequality. The force of static friction is generally less than the coefficient times the normal forceThe force of static friction is generally less than the coefficient times the normal force

4. Starting Motion  If F fr <  s F N =  s mg cos  then the block will hold.  At equality the block just begins to move. m 

5. Measuring Friction: Pulley m1m1 m2m2 F rope FTFT FTFT  Use a pulley and vary the weight to see when movement begins. Forces balance on mass 1 Tension is equal on the rope Forces balance on mass 2

6. Coefficient of Friction: Pulley m1m1 m2m2  The pulley determines the coefficient of friction through the ratio of masses.

7. Measuring Friction: Incline m   Use an incline and vary the angle to see when movement begins. One component of gravity balances the normal forceOne component of gravity balances the normal force The other component of gravity balances frictionThe other component of gravity balances friction

8. Coefficient of Friction: Incline m   The incline determines the coefficient of friction through the tangent of the angle.

9. 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 loadReduce normal force by lightening the load Reduce normal force by adding additional upward forceReduce normal force by adding additional upward force Add a lubricant to reduce the coefficient of frictionAdd a lubricant to reduce the coefficient of friction next