GravityGravityChapter 3 Section 2Chapter 3 Section 2

Effects of GravityEffects of GravityWhy does the apple fall down Why does the apple fall down

from the tree and not float from the tree and not float around in space?around in space?

All objects have forces of All objects have forces of attraction between themattraction between them

this force of attraction is called this force of attraction is called gravitygravity

GravityGravity is the force of attraction is the force of attraction between objects and the Earthbetween objects and the Earth

the amount of gravitational the amount of gravitational force between objects depends force between objects depends on their on their massesmasses and the and the distancedistance between thembetween them

WeightWeightThe force that is caused by The force that is caused by

gravity. The mass and the gravity. The mass and the distance determine the weight.distance determine the weight.

Good test questionGood test questionDo you weigh more, less or the Do you weigh more, less or the

same on the moon compared to same on the moon compared to Earth?Earth?

The Third Law of MotionThe Third Law of Motion

Chapter 3 Section 3

Newton’s Third LawNewton’s Third Law

Shooting a gunShooting a gunrowing a boatrowing a boatrocket propulsionrocket propulsionice skater pushing office skater pushing off

Newton’s third law of motionNewton’s third law of motion–when one object exerts a force on when one object exerts a force on

a second object, the second one a second object, the second one exerts a force on the first that is exerts a force on the first that is equal in size and opposite in equal in size and opposite in directiondirection

–to every action force there is an to every action force there is an opposite and equal reaction forceopposite and equal reaction force

How is this law different from the How is this law different from the other two?other two?

Forces don’t cancel each other out Forces don’t cancel each other out even if they are equal and opposite.even if they are equal and opposite.

There are There are alwaysalways two objects two objects involvedinvolved

If forces are equal then how can a If forces are equal then how can a swimmer move through the water?swimmer move through the water?

Newton’s second law determines Newton’s second law determines the acceleration of an objectthe acceleration of an object

The forces are equal, but The forces are equal, but the masses may not bethe masses may not be

the objects with less mass the objects with less mass will accelerate morewill accelerate more

MomentumMomentumWhy is it harder to stop a Why is it harder to stop a

loaded train car verses an loaded train car verses an unloaded train car?unloaded train car?

Why is it harder to stop a train Why is it harder to stop a train car moving at 10 m/s verses a car moving at 10 m/s verses a train car moving at 5 m/s?train car moving at 5 m/s?

Momentum is a property an Momentum is a property an object has due to its mass and object has due to its mass and velocityvelocity

can be calculated by can be calculated by multiplying mass times velocitymultiplying mass times velocity

momentum = mass x velocitymomentum = mass x velocity = m x v= m x vunits are kgm/s units are kgm/s

Force & Changing MomentumForce & Changing Momentum

How do you stop the baseball from stinging How do you stop the baseball from stinging your hand as you catch it?your hand as you catch it?

Why to air bags work?Why to air bags work? Newton’s 2Newton’s 2ndnd law says F=ma law says F=ma And And

So So

t

vva if

)(t

vvmF if

Rearrange to have the equationRearrange to have the equation Ft=m(vFt=m(vff-v-vii) or Ft=m) or Ft=mΔvΔv

Where: Ft is called impulse Where: Ft is called impulse So the impulse equals the change in So the impulse equals the change in

momemtummomemtum

Momentum has direction Momentum has direction momentum will change only if momentum will change only if

its mass or velocity changesits mass or velocity changesmomentum can be transferred momentum can be transferred

from one object to anotherfrom one object to another–pool ballspool balls–car crashescar crashes–hockey playershockey players

Law of Conservation of Law of Conservation of MomentumMomentum

The total amount of momentum The total amount of momentum of a system does not change of a system does not change unless an outside force acts on unless an outside force acts on the systemthe system–pool ballspool balls