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.
 Fw = m*g
 “g” refers to our gravity
[which we already went over…]
 on Earth, g = 9.8m/s2
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
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
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
 0.057 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
 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).
HOMEWORK
 complete
 page 17
 Newton’s Laws w/s