1. 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. F w = m*g “g” refers to our gravity [which we already went over…] on Earth, g = 9.8m/s 2

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: F w = 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/s 2 Mars, g = 3.7 m/s 2 Jupiter, g = 24.8 m/s 2

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 p 1i + p 2i = p 1f + p 2f (m 1 v i +m 2 v i = m 1 v f +m 2 v f )

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 3 rd Law).

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