Unit 5 – Lecture 5

Newton’s Third Law

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]

Forces and Vectors

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

Gravity

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

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 lbs on Jupiter 47.1 lbs on Mars 8.3 lbs on Pluto

Practice: F w = m*g How much does a 25 kg object weigh in Newtons? 245 N 2.6 N N If an object weighs 397 N, what is its mass? 0.02 kg 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

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

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

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

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

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

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]

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

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.

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 )

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.

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).

HOMEWORK complete page 17 Newton’s Laws w/s