Newton’s Third Law of Motion

Building Science Champions

In thoughthttp://www.wimp.com/spaceshuttle/

After watching the Discovery launch, explain how Newton’s three laws are used by NASA scientist to launch a space shuttle.

Objectives State Newton’s third law of motion

Define and calculate momentum and state the law of conservation of momentum

Key Terms Force pair

Momentum

Law of conservation of momentum

Elastic collision

Inelastic collision

Newton’s Third Law of Motion

Newton realized forces are not one direction

One object exerts a force on the second; while the second exerts a force back on the first

Second’s force is equal to and opposite of the first

First force is action and the second force is reaction

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

IMPORTANT

Force PairsForces in Newton’s 3rd law depend on each other

Force pair – is the force two objects apply to each other

Why don’t equal but opposite forces cancel each other out? Because each force acts on a different object

The person is pushing on the diving board The diving board is

pushing on the person

2 separate forces

Equal but Opposite

Action ForceReaction

Force

Equal

Opposite

When contact between the two players is made the forces are equal.

The players flying backwards is the opposite force.

The ground is the unbalanced force that will stop the players fall.

The force the white player hits the blue player with is transferred to the blue player and that is equal to the force that he will fall with.

Action-Reaction in Action A squid brings water in, then expels it to move

Action expulsion of water

ReactionSquid moves forward A space

shuttle burns fuel to create thrust, a pushing force, the ground pushes back on the shuttle

Reactio

n

Action

Momentum Momentum (p) – is the product of an object’s

mass and it’s velocity

p = mv

Mass is measures in kilograms (kg) and velocity is measured in meters per second (m/s)

Momentum is also described by it’s direction as well as its quantity

Momentum Problem Which has more momentum: a 3-kg

sledgehammer swung at 1.5 m/s or an 4-kg sledgehammer swung at 0.9 m/s?

Momentum = mass x velocity

3 kg x 1.5 m/s = 4.5 kg x m/s

4 kg x 0.9 m/s = 3.6 kg x m/s

Practice Problems

Golf ball = 0.72 kg x m/s baseball = 0.98 kg x m/s

A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has greater momentum?

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

Bird = 0.27 kg x m/s

Conservation of Momentum

Law of conservation of momentum – the total momentum of the objects that interact does not change

Quantity of momentum is the same before and after they interact

Total momentum of any group of objects remains the same unless outside forces act on the objects

Conservation refers to conditions before and after an event

A quantity that is conserved is the same after an event as it was before the event

Types of CollisionsObjects collide with one another in two different ways

Elastic collision – when colliding objects bounce off one another in the collision

Inelastic collision – when colliding objects stick to another

No matter the type of collision, the total momentum will be the same before and after the collision

Two Moving Objects Look at the two trains traveling in the same direction, train 1 is traveling at 10 kmph and train 2 is traveling at 5 kmph

10 kmph

5 kmph

When the trains collide the speed of each car changes

Train 2 is now traveling at 10 kmph and train 1 is now traveling at 5 kmph

10 kmph 5

kmph

Momentum is conserved – the momentum of one train decreases while the momentum of the other increases

One Moving ObjectCue ball is moving toward the stationary billiard balls

Stationary

Moving

The momentum from the cue ball is transferred to all the other billiard balls

Cue ball will become stationary, the stationary billiard balls will go in motion

Stationary

Momentum

The momentum is the same before and after the collision. This time the momentum from the cue ball is transferred to numerous other billiard balls.

Two Connected ObjectsInstead of objects bouncing off of each (elastic collision) other sometimes they will remain connected (inelastic collision)

Player A is running at 4 m/s and player B is stationary

4 m/s

Stationary

Once contact is made, the two players will share the momentum of player A

2 m/s

2 m/s

The momentum of the two players is still conserved

ReviewMomentum’s formula is p=mv

Elastic collisions the objects bounce off one another

Inelastic collisions the objects stay connected

Reactions are opposite but equal to the action

Newton’s third law is action/reaction

References Anderson, M. et all (2012) Physical Science.

McGraw-Hill: Columbus

Frank, D.V et al (2001). Physical Science. Prentice Hall: New Jersey