Section 3.1Newtons Third Law and Momentum

RecapN1: An object in motion remains in motion and an object in rest remains in rest unless it is acted upon by an outside force.

N2: F=ma

N3: ???

Newtons Third LawFor every action there is an equal and opposite reaction.

Ohso thats why when I punch a fool in the face my hand hurts!

Forces ALWAYS come in pairs!The forces are called Action/Reaction PairsThe forces are always equal in magnitude and opposite in directionThe forces always act on different objects

There is no such thing as a force that is alone

They necessarily come in pairs

http://www.youtube.com/watch?v=cP0Bb3WXJ_k

Question?Remember net forces?

Why dont action and reaction forces cancel out?

Because they act on different objects.

Forces could only cancel out if they act on the same object.

Identifying Action/Reaction ForcesBoth are always there whenever any force appearsThey always have the exact same strengthThey always act in opposite directionsThey always act on different objectsBoth are real forces and either (or both) can cause acceleration

Describe an action/reaction pair of forces when

Joe Mama sits in a chair.

Describe an action/reaction pair of forces when

Joe Mama sits in a chair.

The chair exerts a force on Joe, and Joe exerts a force on the chair

The Earth exerts a force on Joe, and Joe exerts a force on Earth

Describe an action/reaction pair of forces when

A baseball player hits a ball with a bat.

Describe an action/reaction pair of forces when

A baseball player hits a ball with a bat.

The bat exerts a force on the ball, and the ball exerts a force on the bat

Thats why the ball accelerates and the bat slows down

Momentum

Choose Your Doom!Would you rather:

Get hit by a snowball going 10 m/sorGet hit by a snowball going 40000000 m/s

??

We know that more massive objects are harder to stop (inertia)

but what about objects with the same mass but different speeds?

MomentumLike inertia, momentum measures an objects resistances to changes in its motion

Unlike inertia, momentum accounts for the effects of speed in addition to mass

Sadly, momentum only measures the resistances to changes in motion of moving objects

Momentum

Changes in MomentumMomentum changes when velocity changes. Since force is what changes velocity, that means that force is also linked to changes in momentum.

A change in momentum by a force over a period of time is called Impulse.

Impulse = Change in Momentum

ProblemA net force of 100 N is applied for 5 s to a 10-kg car that is initially at rest. What is the speed of the car at the end of the 5 s?

ProblemA net force of 100 N is applied for 5 s to a 10-kg car that is initially at rest. What is the speed of the car at the end of the 5 s?

Did you get 50 m/s? Good cause thats the right answer!

ProblemA 15 N force acts for 10 s on a 1-kg ball initially at rest. What is the balls final momentum?

ProblemA 15 N force acts for 10 s on a 1-kg ball initially at rest. What is the balls final momentum?

150 kg.m/s or 150 N.s

ProblemHow much time should a 100-N force take to increase the speed of a 10-kg car from 10 m/s to 100 m/s?

ProblemHow much time should a 100-N force take to increase the speed of a 10-kg car from 10 m/s to 100 m/s?

9 s

Law of Conservation of MomentumN3: Equal and opposite forcescause an acceleration, and thus change the momentum of an object.

From this, changes in momentum must also be equal and opposite.

This is the Law of Conservation of Momentum

Law of Conservation of MomentumIf interacting objects in a system are not acted on by outside forces, the total amount of momentum in the system cannot change.

Forces outside the system, such as friction and gravity, can change the total momentum of the system.

ProblemAn astronaut floating in space throws a 2-kg hammer to the left at 15 m/s. If the astronauts mass is 60 kg, how fast does the astronaut float to the right after throwing the hammer?

ProblemAn astronaut floating in space throws a 2-kg hammer to the left at 15 m/s. If the astronauts mass is 60 kg, how fast does the astronaut float to the right after throwing the hammer?

Answer: 0.5 m/s

ReviewDescribe action/reaction pairs.Why dont action and reaction forces cancel?Use impulse to explain how force is related to changes in momentum.Explain the law of conservation of momentum and how it relates to Newtons third law.