A Force that Opposes Motion Section 1.3: Friction A Force that Opposes Motion

The Source of Friction The amount of friction depends on many factors, which is the force pushing the surfaces together & the roughness of the surfaces. When two surfaces are in contact, the hills & valleys of the surfaces “stick” together

The Effect of force on Friction Amount of friction depends on the force pushing the surfaces together Objects that weigh less, exert less downward force Changing the amount of surface contact does NOT change the amount of friction

The Effect of Rougher surfaces on Friction Rough surfaces have more microscopic hills and valleys than smooth surfaces Friction is greater for rougher surfaces

Kinetic Friction Friction between moving surfaces Surfaces can slide past each other or a surface can roll over another surface

Sliding Kinetic Friction Usually a greater force than rolling kinetic friction Examples: applying brakes on a bike, writing with a pencil, scratching an itch

Rolling Kinetic Friction It is easier to move objects on wheels, than by sliding Important part of almost all means of transportation examples: bicycles, skating, trains, cars, etc.

Static Friction Static means not moving When a force is applied to an object, but the object does not move, static friction occurs Static friction disappears as soon as an object starts moving Kinetic friction immediately occurs Applying a large enough force can overcome static friction

Helpful Car tires could not push against the ground to move the car Brakes could not stop the car You would slip and fall when you tried to walk

Harmful Moving parts in a car’s engine wear down Wind and water can cause erosion of soil Socks and jeans develop holes (wear out)

Reducing Friction Use lubricants between surfaces (oil, grease, wax) Switch from sliding kinetic friction to rolling kinetic friction Make surfaces that rub against each other smoother

Increasing Friction Make surfaces rougher Example: sand on icy roads Increase the force pushing surfaces together Example: pressing harder when sanding wood