5.3 Friction and Normal Force

5.3 Friction and Normal Forcepp. 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

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