Newton’s Third Law

1. Newton’s Third Law Unit 5 – Lecture 5

2. Newton’s Third Law

3. Newton’s Third Law – cont’d • For every action, there is an equal but opposite reaction. • these forces are acting on different objects, so they are not balanced forces[balanced forces act on the same object]

4. Forces and Vectors

5. Gravity • Gravity - a force of attraction between all objects which have mass • it is inherent to all matter • depends on: • distance between the masses • “inverse square” law – physical quantity or strength is inversely proprotional to the square of the distance from that source of the physical quantity • size of the masses

6. Gravity

7. Mass vs. Weight • Mass – the measure of the amount of matter in an object. • measures the inertia of an object [more mass, more inertia / less mass, less inertia] • Weight – the force of attraction caused by gravity acting on a mass. • Fw = m*g • “g” refers to our gravity [which we already went over…] • on Earth, g = 9.8m/s2

8. Mass vs. Weight – cont’d • We use weight and mass interchangeably because the only comparison we have is the Earth’s gravity. • Weight will change based on local gravity; NASA has to take this into effect • example: Susie weighs 125 lbs on Earth. She weighs: • 20.7 lbs on the moon • 295.5 lbs on Jupiter • 47.1 lbs on Mars • 8.3 lbs on Pluto

9. Practice: Fw = m*g • How much does a 25 kg object weigh in Newtons? • 245 N 2.6 N 0.392 N • If an object weighs 397 N, what is its mass? • 0.02 kg 3890.6 kg 40.5 kg • If an object weighs 1100 N but has a mass of 125 kg, which planet is it on? • Venus, g = 8.8 m/s2 Mars, g = 3.7 m/s2 Jupiter, g = 24.8 m/s2

10. Friction • Friction is the force that opposes all motion. • A moving object will always lose energy to friction

11. Friction – cont’d • Three Types of Friction: • Sliding Friction • when two surfaces slide along each other • most resistive friction / requires the most force • can lead to buildup of heat

12. Friction – cont’d • Three Types of Friction: • Rolling Friction • when an object or surface rolls along another • less resistive than sliding, more resistive than fluid

13. Friction – cont’d • Three Types of Friction: • Fluid Friction • solid surfaces sliding with a layer of liquid or gas between them[layer fills in the tiny depressions on surfaces • least resistive friction – requires the least force • lubrication • air resistance

14. Practice • What type of friction is exhibited by your shoes on the ground? • sliding • What type of friction is exhibited by a lubricant? • fluid • What type of friction is exhibited by the wheels on a cart? • rolling

15. Momentum • Momentum - the product of an object’s mass multiplied by its velocity • p [momentum] = m [mass] * v [velocity] • p = kg * (m/s) = kg*m/s [kilogram meters per second]

16. Practice • What is the momentum of a 210 kg hog running at 12 m/s? • 17.5 kg*m/s • 2520 kg*m/s • 0.057 kg*m/s

17. Momentum – cont’d • Law of Conservation of Momentum – momentum can not be created or destroyed under normal circumstances • it can be changed from one form to another • total momentum before = total momentum after • ex: pieces of a dropped light bulb, etc.

18. Law of Conservation of Momentum • Formula • object 1 initial momentum + object 2 initial momentum equals • object 1 final momentum + object 2 final momentum • p1i+ p2i = p1f + p2f (m1vi+m2vi = m1vf+m2vf)

19. Conservation of Momentum &Newton’s Laws • An untethered astronaut is stranded away from his spaceship while working on a satellite in space. The only equipment he has is all the tools he was using to repair the satellite. • Discuss with a partner what he could do – according to these two laws – to get back to the ship.

20. Conservation of Momentum & Newton’s Laws • If the astronaut were to throw a piece of equipment away from the spaceship, he would gain a small amount of momentum in the direction of the spaceship (Newton’s 3rd Law).

21. HOMEWORK • complete • page 17 • Newton’s Laws w/s