FRICTION The force between two objects in contact that opposes the motion of either object. Friction is oppositional or resistance force Friction depends upon surface roughness material type the degree to which the surfaces are pressed together. Friction occurs as the irregularities between surfaces settle together, forming microwelds

A CLOSER LOOK: SANDPAPER Friction is caused by the irregularities in the surfaces of objects that are touching. (Even very smooth surfaces have microscopic irregularities that obstruct motion.) The rougher the surface, the ______________ the friction. greater

Friction Two types of friction Static (not moving) Kinetic (sliding) Rolling (reduces surface areas in contact and reduces friction)

Static Friction Force that prevents motion Abbreviated Fs How does the applied force (F) compare to the frictional force (Fs)? (it is the same in magnitude and builds until static friction force is broken) Static friction force is greater than kinetic friction force These questions should help students understand that static friction balances the external force (F), so it increases and decreases as F increases and decreases. Eventually, F will be so large that the static frictional force (Fs) will no longer be able to balance it, and the net force will cause the object to slide. At this point, frictional forces become kinetic (see next slide).

Kinetic Friction Force between surfaces that opposes movement Abbreviated Fk Does not depend on the speed Using the picture, describe the motion you would observe. The jug will accelerate. How could the person push the jug at a constant speed? Reduce F so it equals Fk. (applied force equals kinetic friction force at a constant velocity) Ask students if it requires more force to get an object moving when it is at rest or to keep it moving once it is already in motion. When pushing an object, we exert enough force to overcome static friction. At that point the object moves. The opposing force is now kinetic friction, which is less than static friction. Therefore, in order to maintain a constant speed and not accelerate, the force pushing the object is reduced.