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