Newton’s ______ Law:

In plain words:

For every_____________ ,

there is an equal and opposite____________.

In physics speak:

If object A exerts a __________ (the action) on

object B, then object B exerts a ___________ (the

reaction) that is ________________________ but

__________________________ on object A.”

A BF-F

actionreaction

actionreaction

3rd

force F

force –Fequal in magnitude

opposite in direction

action:rocket pushesgases backward

re-action:gases pushrocket forward

action:runnerpushesgroundbackward

re-action:ground pushesrunner forward

NP

View of Earth from North Pole:

walking east

force of footon ground

Will Earth speed up or slow down?

What if they walk west?

force of footon ground

counterclockwiserotation

Ex. Punching things:

Which object exerts a greater force?

Which object exerts a greater force?

F-F

F-F

force of fiston paper:

force of paperon fist:

force of fiston block:

force of blockon fist:

Action-reaction forces are ______________ in case 2, but both cases are opposite and _________ .

Case 1: punching paper

Case 2: punching a cement block

greaterequal

neither

neither

Forces ALWAYS occur in __________ .

Either force could be called the ___________.

One force does not happen _____________

Both forces occur at the ___________________.

Ex 1: F: Earth ________ you down with _________.

-F: You _______ Earth up with _________.

pairs

action“first.”

same time

type

Both forces are ____________ .

Both forces are ____________

Notice the "same types:"

pullsgravity

pulls gravity

pull gravity

Fg

Fg

A/ Both forces are the exact same ___________ .

Ex 2: F: You ________ right on wall with a __________________. -F: Wall __________ left on you with a __________________.

Ex 3: F: Wire ________ up on picture with a _______________. -F: Picture _________ down on wire with a________________.

normal forcepush

normal force.

pushes

wall

hand

FNFN

pullstension

pullstension

T

T

B/ Action:reaction pairs act on ______________ objects in _____________ directions.

Action: A exerts a F on B.

Reaction: ___ exerts a -F on ____.

Ex 1: F: Earth pulls you down with gravity.

-F: ______ pull _______ ____ with gravity.

B A

different

You Earth up

subject object direction

subject object____________direction

Notice how subject and object are _____________ ,

and the directions are ________________ .

opposite

reversed opposite

opposite

Ex 2:

F: You push right on wall with a normal force.

-F: ______ pushes ______ on ______ with a

normal force.Ex 3:

F: Wire pulls up on picture with a tension.

-F: __________ pulls _________ on ________with

a tension.

Wall left you

Picture down wire

In Sum: To find a reaction force, re-write with:1. the same _________ of force (push/pull

and gravity/normal/tension, friction, etc); but,2. reverse _____________ and _____________ and the _______________ of the force.

type

subject objectdirection

"A (subject)

pulls/pushes

in a certain direction

on

B (object)

with a certain force."

Action: Reaction:

" ____ (subject)

_________________

in ___________ direction

on

___ (object)

with _____________force."

B

pulls/pushes

opposite

A

the same

Which force, action or reaction, has a greatermagnitude? neither

Ex: Write the action (A) and reaction (R) pairs.

block

table

Case 1/ Between block and table:

A:

R:

Case 2/ Between block and Earth:

A:

R:

Which of these are contact forces?Which are "at a distance" forces?

Block pushes down on table with normal force.

Table pushes up on block with normal force.

Block pulls up on Earth with gravitational force.

Earth pulls down on block with gravitational force.

case 1

case 2

Ex: Weight is the force of Earth’s ___________ pulling ______________on an object:

w = ____ = Earth pulls__________ on cat with a ________________ force.

What is the reaction force to acat's weight, w?

w

w

w

w

wThe _______ _______ _______on ________ with a__________________ force.”

gravity downward

gravitational

downwardFg

cat pulls upEarth

gravitational

Ex. Don’t confuse ___________ (which are always equal and opposite) with ___________________ (which depend on _________ and usually are NOT equal and opposite).

Ex: An apple and the Earth (not to scale)

gravitational F ofEarth on ________

gravitational F of apple on _________

apple: a = 9.81 m s2

m ≈

forcesaccelerations

mass

m

F=

Earth: a = F0

Same _____ but different _____ b/c different _____ !F's a's m's

apple

Earth

Open your 3-ring binder to theWorksheet Table of Contents.

Record the title of the worksheet:

Newton’s 3rd Law WS

box

Earth

box

Earth

50 N50 N

100 N

100 N

80 N50 N

100 N

100 N

When determining if an object accelerates,you do NOT add in the reaction forces.