1. Newton’s first law says that an object in motion stays in motion, but if you roll a ball down a hallway it stops.  Why?
2. What factors does friction depend on?
3. What causes friction?
4. If you push on a box with a force of 10N and it doesn’t move, what is the frictional force acting on it?
5. Why is it easier to pull a load on a wagon than drag it on the floor?
6. When would a feather and an apple fall at the same speed?

 

Friction—a force between two objects that are touching that resists motion

Ø   Friction is caused by the roughness of the two surfaces sticking together—even if they look smooth, they aren’t!

Ø   Friction depends only on mass and roughness of the surfaces, not surface area

Ø   Static friction—friction between 2 surfaces that are not moving (ex. pushing a box that is too heavy to move)

·       No motion = balanced forces

·       frictional force is equal to the force being applied

Ø   Sliding friction—friction between 2 surfaces that are sliding past each other (ex. pushing a box across the room)

·       sliding friction is less than static friction

Ø   Rolling friction—friction between a rolling object and the surface it rolls over (ex. train wheels on a track)

·       rolling friction is very small—much less than other types

 

v             Air resistance is like friction with the air; it opposes gravity and slows falling objects.  It depends on surface area