Unit 5 – Lecture 5

Newton’s Third Law

Newton’s Third Law – cont’dFor 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

GravityGravity - a force of attraction between all

objects which have massit is inherent to all matterdepends 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. WeightMass – 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

Mass vs. Weight – cont’dWe 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 moon295.5 lbs on Jupiter47.1 lbs on Mars8.3 lbs on Pluto

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

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

friction

Friction – cont’dThree Types of Friction:

Sliding Frictionwhen two surfaces slide along each othermost resistive friction / requires the most

forcecan lead to buildup of heat

Friction – cont’dThree Types of Friction:

Rolling Frictionwhen an object or surface rolls along

anotherless resistive than sliding, more resistive

than fluid

Friction – cont’dThree Types of Friction:

Fluid Frictionsolid 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 forcelubricationair resistance

PracticeWhat 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

MomentumMomentum - the product of an object’s mass

multiplied by its velocityp [momentum] = m [mass] * v [velocity]p = kg * (m/s) = kg*m/s [kilogram meters

per second]

PracticeWhat is the momentum of a 210 kg hog

running at 12 m/s?17.5 kg*m/s2520 kg*m/s0.057 kg*m/s

Momentum – cont’dLaw of Conservation of Momentum –

momentum can not be created or destroyed under normal circumstancesit can be changed from one form to anothertotal momentum before = total momentum

afterex: pieces of a dropped light bulb, etc.

Law of Conservation of MomentumFormula

object 1 initial momentum + object 2 initial momentum

equalsobject 1 final momentum + object 2 final

momentum

p1i+ p2i = p1f + p2f

(m1vi+m2vi = m1vf+m2vf)

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 3rd Law).

HOMEWORKcomplete

page 17Newton’s Laws w/s