bellringer In your composition books Draw a free-body diagram of you sitting on a chair (make-sure to include all the forces) What is a normal force? Define and draw a picture

5.2 friction Davis

What is friction A force that resists motion. Found everywhere in our world Can act when an object is moving or when it is at rest. Depends on both the surfaces that are in contact

Identifying friction of forces Draw force of friction with a force vector Friction is measured in Newtons The force of friction acting on a surface ALWAYS points OPPOSITE the direction of the motion of that surface

Types of friction Sliding friction: friction forces that resist the motion of an object moving across a surface

Types of friction cont. Static friction: the friction force that resist the motion between two surfaces that are not moving As you increase the strength of your push, the static friction also increase (eventually your force becomes strong enough to overcome static friction and the box starts moving) Static friction is almost ALWAYS GREATER than sliding friction

Different amounts of friction The amount of friction generated when a piece of paper is slide across a table vs. adding a brick to the piece of paper than sliding it across the table is different. The greater the force squeezing two surfaces together, the greater the friction force

Decreasing friction Lubricants Oil: can be used between 2 sliding surfaces to keep them from touching each other. Example: you add oil to a car’s engine so that the moving parts slide or turn with less friction Water Ball bearings Separating the 2 surfaces with a cushion of air Example: metrorail (the train is separated from the train tracks maglev which allows the train to float on the tracks when it is moving this allows the ride t be smoother and faster)

Increasing friction Brakes on a bicycle create friction between the brake pads and the rim of the wheel making the bicycle slow down or stop Tires of a car or any vehicle Cleated shoes are designed to increase the friction between your foot and the ground

Friction and energy Friction changes energy of motion into heat energy (rubbing hands together causes your hands to get warm) Friction is ALWAYS present in any machine with MOVING PARTS (example: sewing machine) Another way friction changes energy is by wearing away moving parts (example: water flowing over stones) each time 2 moving surfaces touch each other , tiny bits of materials are broken off by friction- this uses energy and causes the rocks to be round and smooth