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: 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
m q Starting Motion • If Ffr < msFN = msmg cosq, then the block will hold. • At equality the block just begins to move.
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 m1 m2 Measuring Friction: Pulley FT Frope FT Frope
The pulley determines the coefficient of friction through the ratio of masses. m1 m2 Coefficient of Friction: Pulley
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 m q Measuring Friction: Incline
The incline determines the coefficient of friction through the tangent of the angle. m q Coefficient of Friction: Incline
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 next