KEY CONCEPTS

•WHAT IS NEWTON’S THIRD LAW OF MOTION?•HOW CAN YOU DETERMINE THE MOMENTUM OF AN OBJECT?•WHAT IS THE LAW OF CONSERVATION OF MOMENTUM?

KEY TERMS:•MOMENTUM

Newton’s Third Law of Motion

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Newton’s Third Law of Motion

If one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object

“For every action there is an equal but opposite reaction”

Action & Reaction

Action forces produce the interaction, and the reaction force must be in existence at the same time, have the same size, and be opposite in direction

You apply a 10 N eastward force to wall, wall applies a 10 N westward force to you

Examples

Swimming, rowing

Swimmer pushes backward on the water…… the water pushes swimmer forward

The oar pushes water backward… water pushes oar and boat forward

Do Action – Reaction Forces Cancel?

If 2 equal forces act in opposite directions on the same object they are balanced and cancel each other out. There is no movement

Action – Reaction forces do not cancel out because they are acting on 2 different objects

Forces on different masses

Cannon and cannonballDo both the cannon and the cannonball experience the same force the cannon is fired? Yes, the force on each are the Action/Reaction pairs

Does the cannon or the cannon ball have a larger acceleration? The ball, b/c it has a small mass. It is easier to accel. a small mass

a = F/m

Detecting Motion

You can’t always detect motion when paired forces are in action.

If one of the pairs is much more massive, you will only see the less massive object move

Example – When you push down on the Earth, you won’t see the Earth move, only you jumping in the air

oppositely directed force on the first one.

If the example above

Normal Force

“Normal” means perpendicular Force of the Earth (ground) pushing up on an object when they are in contact with each other

If object is on a horizontal surface and no other vertical forces are applied, then Normal Force is Equal to the weight (force of gravity) of the object

Normal Force is NOT the reaction to the force of gravity acting on an object

Responsible for how heavy we “feel”AKA “Apparent Weight”

Momentum

A characteristic of a moving object that is related to the mass and velocity of the object

Can be determined by the following formula”

Momentum = mass X VelocityUnit is kg *m/sLike velocity and acceleration, momentum has a direction. Its direction is the same as its velocity

Sample Momentum Problem

Which has more momentum: a 3.0 kg sledgehammer swung at 1.5m/s, or a 4.0 kg sledgehammer swung at .9m/s?

Mass 1 = 3.0kgVelocity 1 = 1.5 m/sMomentum = mass X velocity = 3.0kg * 1.5m/2 = 4.5kg*m/s

Mass 2 = 4.0kgVelocity 2 = .9m/sMomentum = mass X velocity = 4.0kg * .9m/s = 3.6kg*m/s

Math Practice 1

A golf ball travels at 16m/s, while a baseball moves at 7 m/s. The mass of the golf ball is .045 kg and the mass of the baseball is 0.14 kg. Which has greater momentum?

Math Practice 2

What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s?

Momentum

The more momentum an object has the harder it is to stop

The more mass an object has, the more momentum. You can stop a baseball traveling 20m/s, but not a truck

The greater the velocity an object has, the harder it is to stop. An arrow shot from a bow has a large momentum even though it has a small mass

Conservation of Momentum

Conservation – same amount before and after an event

Total amount of momentum objects have when they collide is conserved

Law of Conservation of Momentum – states that in the absence of outside forces, the total momentum of objects that interact does not change

Friction is an example of an outside force

Collision with Moving Objects

Sources

http://www.sciencetoymaker.org/balloon/images/newton.gif

http://onlinephys.com/newton3.htmlhttp://www.calctool.org/CALC/phys/newtonian/momentum.png

http://www.toyday.co.uk/shop/images/uploads/bow-and-arrow-set-3.jpg