1. Friction January 28, 2016 7 th grade Chapter 3.5

2. Gravity If you drop something from an airplane, it will fall faster and faster until it reaches a certain point It is getting faster, so it’s accelerating It’s accelerating because of gravity On earth, gravity is always ~10 m/s 2 o It changes if you’re really high up (9.81 m/s 2 vs 9.79 m/s 2 in Flagstaff) On the moon, gravity is ~1.6 m/s 2

3. Gravity is a Force Gravity is a force Force: a push or pull described by strength and direction of how it acts. Measured in newtons (N) Can be direct like 2 objects touching Can be indirect like gravity or magnets Newton is an SI unit (kg·m/s 2 )  Has a name of its own, but can be traced back to base SI units

4. Gravity is Everywhere Everything with mass has gravity, but usually it’s so little that we just call it 0 gravity Gravity is affected by mass and distance What is the difference between mass and weight?

5. Force Force = mass x acceleration

6. Net Force Mass is always +, but acceleration can be + or – That means force can be + or – too Net force: All of the forces acting on an object added up If an object feels a non-zero net force it will experience motion

7. Net Force Will the log move?

8. Net Force What forces might be acting here?

9. Friction Friction: the force that 2 surfaces exert on one another It’s easier to move across or stick to some surfaces than others Normal person: how easy it is to slide Scientist: 4 types of friction depending on movement & state of matter

10. Friction High FrictionLow Friction

11. 4 Types of Friction Sliding Friction: The force exerted when 2 solid surfaces slide across each other Static Friction: The force when 2 solid surfaces are stuck together and not moving (yet) Fluid Friction: The force when a solid object moves through a liquid or gas (fluid) Rolling Friction: The force when a solid object rolls across a surface

12. Classify These Examples A tow truck pulls a car in neutral vs with brakes on Skates make a tiny layer of water on the ice A box is shoved along carpet A box is shoved along tile A really heavy box doesn’t move until 2 people push it Rubber shoes help you climb a vertical pole You slide down a fireman’s pole You go swimming in water vs syrup A paper airplane is thrown You rub your hands together and they get warm