Newton's Third Law

of Motion

1. FORCES AND INTERACTIONS1. FORCES AND INTERACTIONS

Force is an interaction between one thing and another.

Ex: 2 people pushing each other on skate boards, driving a nail, punching a bag

There is no single isolated force

Law 3 - Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

Action-reaction pairs never act on same object.

2. NEWTON'S THIRD LAW OF MOTION

2. NEWTON'S THIRD LAW OF MOTION

Newton’s Third Law deals with Newton’s Third Law deals with

acting on objects.acting on objects.

two forcestwo forces

two differenttwo different

Newton’s Third Law pairs of forcesNewton’s Third Law pairs of forces

alwaysalways, , sometimessometimes, , nevernever

cancel each other out.cancel each other out.

The The only wayonly way for two forces to cancel for two forces to canceleach other out is for them to be each other out is for them to be equalequal

and oppositeand opposite and act on the and act on the samesame object! object!

Action:earth pulls you

Reaction: you pull earth

Action & Reaction on Different Masses

Consider you and the earth

You F m =

F m=a a

Earth

Action: tire pushes on road

Reaction: road pushes on tire

Action: rocket pushes on gases

Reaction: gases push on rocket

  Hwk read Review book 82-86 do pg 100    #4,5, 8 – 14, 24 - 27, 31-35. Separate sheet.

Newton’s 3rd Law Six minutesNewton’s 3rd Law Six minutes

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

The subscript notation:The subscript notation:

Convention: FFa,b means “the force acting on a due to b”.

So FFhead,thumb means “the force on the head due to the thumb”.

FFhead,thumb

Newton's Third Law says...

FFA ,B = - FFB ,A.

FFm,w FFw,m

FFm,f

FFf,m

Hmmm…. The man leans on the box, the box moves.

Since FFm,b = -FFb,m, why isn’t FFnet = 0 and aa = 0 ?

a ??a ??

FFm,b FFb,m

ice

Need free body diagram of box only.

Consider only the box only the box as the system!

–FFon box = maabox = FFb,m

aaboxbox

FFm,bFFb,m

ice

The forces acting on the box are:

1. The force of the man2. The force of the ice (neg).

The forces acting on the box are:

1. The force of the man2. The force of the ice (neg).

aaboxbox

3. SUMMARY OF NEWTON'S THREE LAWS

3. SUMMARY OF NEWTON'S THREE LAWS

Law 1 (Law of Inertia) Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.

amF

Law 2

Law 3 - Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

Newton’s 3rd law …6 minutesNewton’s 3rd law …6 minutes

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

Newton’s 3 Laws 4 min http://www.youtube.com/watch?v=iH48Lc7wq0U

http://www.youtube.com/watch?v=8bTdMmNZm2M&list=PLEA0D7C501E484B31&index=8

Review Questions

A bug and a car collide. Which experiences the greater force?A bug and a car collide. Which experiences the greater force?

(a) bug

(b) car

(c) neither, they both experience the same magnitude of force

(c) neither, they both experience the same magnitude of force

Consider hitting a baseball with a bat. If we call the force applied to the ball by the bat the action force, identify the reaction force.

Consider hitting a baseball with a bat. If we call the force applied to the ball by the bat the action force, identify the reaction force.

(a) the force applied to the bat by the hands

(b) the force applied to the bat by the ball

(c) the force the ball carries with it in flight

(d) the centrifugal force in the swing

(b) the force applied to the bat by the ball

Two blocks are stacked on the ground.

How many action-reaction pairs of forces are present in this system?

(a) 2

(b) 3

(c) 4

a

b

Solution:

(c) 4

FFE,aE,a

FFa,Ea,E

a

b

FFE,bE,b

a

bFFb,Eb,E

FFb,ab,a

FFa,ba,b

a

b

FFg,bg,b

FFb,gb,g

a

b