Friction Friction is a force that opposes motion between two surfaces that are in contact Frictions occurs because surfaces are covered with microscopic hills and valleys. (Even surfaces that feel smooth) The hills and valleys on one surface stick to the hills and valleys of the other surface

2 Factors that affect the amount of friction Amount of force pushing the surfaces together Ex. Heavy and light book Roughness on the surfaces Ex. Rubbing sandpaper pieces together

Kinetic vs. Static Friction Kinetic friction is friction between moving surfaces Static is friction is when a force is applied to move an object but it doesn’t move Static friction disappears once the object starts to move

Ways to reduce friction Apply a lubricant (oil, wax grease) Change from sliding kinetic friction to rolling kinetic friction

Ways to Increase Friction Make surface rougher Increase the force pushing the surfaces together