Friction• Every day you see

moving objects come to a stop.

• The force that brings nearly everything to a stop is friction

• FRICTION is the force that acts to resist movement between two touching surfaces.

• Objects cannot move with a constant speed unless a net force is applied because friction is always acting to slow down the motion

• If friction could be removed, an object in motion would continue to move in a straight line with constant speed.

Friction, continued

• In the vacuum of outer space, there is no friction to slow motion • Therefore objects continue to move indefinitely (like the moon around the earth)

Friction, continued

• If two objects are in contact, frictional forces always try to prevent one object from sliding on the other object.

• If you rub your hand against a tabletop, you can feel the friction push against the motion of your hand.

• Friction will always slow an object down.

Opposing Sliding

• STATIC FRICTION prevents an object from moving when a force is applied

Static Friction

• It is caused by the attraction between atoms on the 2 touching surfaces which cause the surfaces to stick together

• Generally, the rougher the surface or the heavier the object, the more static friction.

• To move the object, you have to exert a force large enough to break the bonds holding two surfaces together.

Static Friction, continued

• SLIDING FRICTION slows down an object that slides.

• You have to keep pushing to overcome the force of sliding friction.

Sliding Friction

• A force must be applied to move the rough areas of one surface past the rough areas of the other.

Sliding Friction, continued

• Sliding friction is due to the microscopic roughness of two surfaces.

• What are some examples of sliding friction you encounter in your everyday life?

• ROLLING FRICTION, occurs between the ground and the part of the tire touching the ground.

Rolling Friction

• Rolling friction keeps the tire from slipping on the ground and exerts force on the tires that pushes the object forward.

• It’s usually easier to pull a load on a wagon or cart that has wheels rather than drag the load along the ground.

Rolling Friction, continued

• Rolling friction between the wheels and the ground is LESS than the sliding friction between the load and the ground.

• Why does a cart have wheels?

Fluid Friction

• FLUID FRICTION occurs between an object and air or water.

• Examples:

• skydivers jump out of a plane

• boat traveling through water

ANSWER THE FOLLOWING

SELF CHECK QUESTIONS:

Question

List and give an example of the four types of friction:

Question

List and give an example of the four types of friction:

Answer

• Rolling Friction (roller skates)• Sliding Friction (plate sliding across a table)• Static Friction (pushing a desk)• Fluid Friction (a person swimming through the water)