1. MOTION AND FORCE Chapter 11 section 3

2. Force The definition of force is “any action that can change the state of motion of an object.” BETTER definition: a push or a pull Four fundamental forces in nature: Gravity Electromagnetic force Strong nuclear force Weak nuclear force Field forces

3. NET force Net force is the combination of all forces that are acting on an object. Forces have direction! This is important when combining forces! If forces add up to zero, then the forces are balanced = EQUILIBRIUM If forces add up to a number, then the forces are unbalanced. Unbalanced forces cause acceleration!

4. Net Force 3 people are fighting over the yellow box. Who wins? 500N 320N 50N ΣF = +500 + -50 + -320 ΣF = 500 – 370 = 130N 500N wins!

5. Another example The net force acting on an object is 40N. With what force is Mary pulling? F John = 300N F Kerri = 500N F Bob = 200N F Mary ΣF = 40N ΣF = F Mary + F John + F Bob + F Kerri 40 = F Mary + 300 + -200 + -500 F Mary = 440N

6. Force of Friction Friction is a force that opposes motion 2 surfaces have to be in contact with each other Ex. Tires and the road 2 kinds: Static and Kinetic Static friction: force that resists motion from rest Kinetic friction: force that resists motion when an object is in motion

7. Why do we get friction? Friction arises from tiny microscopic irregularities that all surfaces contain

8. Sometimes we don’t want friction In engines, between moving parts In joints, between bones Ways to reduce friction: Lubricants: oil, grease, wax How does an air-hockey table work to reduce friction?

9. Sometimes friction is good Trying to walk Paper weights Ways to increase friction Traction: soles of shoes, tire treads: increase contact between surfaces Quarterbacks wear a glove so that he can hold onto the football better What causes a car to move?