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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