# 5.3 Friction and Normal Force pp. 119 - 123 Mr. Richter.

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5.3 Friction and Normal Force pp. 119 - 123 Mr. Richter

Agenda  Warm-Up  Review HW  Notes:  What is Friction?  Kinetic vs. Static Friction  Friction and the Normal Force  Avoiding and Using Friction

Objectives: We Will Be Able To…  Distinguish between kinetic and static friction  Explain the cause of friction  Discuss reasons to increase or decrease friction.

Warm-Up:  Timmy and Tommy are holding a box that weighs 700 N. If Timmy can lift with a force of 450 N, how much force does Tommy have to lift with to keep the box at the same height?

Friction

 Friction is a force that resists the motion of objects.  Friction is caused by the microscopic hills and valleys between two surfaces.

The Direction of Friction  Friction is a force.  On a free-body diagram, friction always points in the direction opposite the motion of the object. Even if the object doesn’t actually move! Force of Friction

Two Types of Friction  Kinetic Friction (“sliding” friction) is present when two objects or surfaces slide across each other.  Static Friction is present when forces are acting to cause an object to move but friction is keeping the object from moving.

Two Types of Friction  The maximum static friction is always greater than the kinetic friction between two surfaces.  This is because the little hills and valleys get stuck together when an object is stopped, but they can skim across each other when an object is sliding.  Think about running your fingers along a chain link fence.

What does Friction Depend on?

Friction Depends On…  The normal force is the force of a surface pushing back on an object.  The word “normal” means perpendicular. The normal force is perpendicular (makes a 90 degree angle) to the surface.  If a piece of paper rests on a table, the table exerts a normal force on the piece of paper.

Friction Depends On…  Friction is proportional to the normal force. Meaning…  The greater the force squeezing two surfaces together, the greater the friction force.

Friction Depends on…  Friction also depends on the two surfaces that are trying to slide against each other.  A hockey puck sliding on ice will work better than a hockey puck sliding on sand paper.

Avoiding and Using Friction

Avoiding Friction  Friction exists any time two surfaces rub against each other.  Friction slows motion, causes wear and tear, and turns useful energy into less useful forms, like heat.  Humans use many things to decrease the effect of friction:  ball bearings  oil  magnets  etc.

Using Friction  Sometimes we want surface to resist motion.  We want tires to stick to the road.  We want nails to stick into the wood.

Wrap-Up: Did we meet our objectives?  Distinguish between kinetic and static friction  Explain the cause of friction  Discuss reasons to increase or decrease friction.

Homework  p123 #1-4  p116 #a,b

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