2. Gravity, Weight, Mass, Friction Friction is a drag!

3. Gravity 4 Gravity is a force that pulls every mass toward every other mass. 4 Earth is the biggest mass around, so gravity pulls everything toward the center of Earth. 4 Gravity depends on mass 4 Every kilogram on Earth creates a gravitational force of 9.8 N

4. Mass and Weight 4W4Weight is the force created by gravity on objects. Mass is the amount of matter in an object. 4W4Weight depends on an object’s mass. 4M4Mass is constant (short term) in whole universe; weight changes depending on where you are in the universe. 4Y4You weigh less on the moon (by 1/6) because gravity on moon is 1/6 that of Earth’s.

5. Mass and Weight, continued 4 Weight is a force, so it’s measured in newtons. 4 F = m a (Newton’s 2nd law) 4 F w = m g (where g = accel. due to gravity or 9.8 m/s 2 ) 4 This is how we calculate mass from weight and weight from mass. 4 If your mass is 60 kg, then what is your weight?

6. Friction 4 Friction describes forces that result from relative motion between objects. 4 Frictional forces always work against the motion that produces them. 4 Friction comes from two surfaces moving against each other. 4 Objects that continuously rub against each other cause wear.

7. Types of friction 4A4Air Friction is caused by the air moving around moving objects. 4S4Sliding Friction is caused by irregularities in the surfaces of two objects. 4V4Viscous Friction is caused by objects moving in water or other fluids. 4R4Rolling Friction is caused by one object rolling over another.

8. What kind of friction? 4 Tractor-trailers now have a shape on the cab. 4 Lazy susans (rotating trays) use ball bearings to help them reduce friction. 4 Pistons in a car are oiled to reduce wear. 4 To remove a tight ring, sometimes the wearer uses lotion. 4 It’s more fun to ice skate on a freshly surfaced rink than on one that has been skated on for awhile.