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 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)

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

Coefficient of Friction m1m1 FTFT  Use rope and measure the force when movement begins. Measure weight Measure force at slipping point.

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

Sliding  Sliding objects also have a frictional force exerted on them.  This frictional force is kinetic friction.  An approximate formula:  k is the coefficient of kinetic friction

Static vs. Kinetic  Static and kinetic friction are similar. Force in opposite direction to motionForce in opposite direction to motion Proportional to normal forceProportional to normal force Coefficient of friction depends on materialsCoefficient of friction depends on materials  Static and kinetic friction are different. Static friction is an inequality up to a maximum Coefficient of friction is typically greater for static friction at the same surface Wood on wood (0.4 vs. 0.2)

Downhill Skiing  A TV station has invited you to be the science commentator for an upcoming ski race. You observe that a skier needs at least 3° of slope to move forward at a constant velocity.  What is the coefficient of friction on the skis?  °

Force Diagram  At constant velocity the forces must all balance.  Friction doesn’t act in the direction of the normal force.  The normal force cancels the component of gravity.  F g = -mg F gy = -mg cos  F N = mg cos  F fr = -  k F N

Minimum Sliding  When the skier is sliding slowly we can neglect air resistance.  Kinetic friction balances the component of gravity pulling forward.  ° F gx = mg sin  F N = mg cos  F fr = -  k F N

Normal Force and Friction  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