1. For every_____________ , there is an equal and opposite____________.
Newton’s ______ Law:
In plain words:
For every_____________ ,
there is an equal and opposite____________.
3rd
action
reaction
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.”
force F
force –F
equal in magnitude
opposite in direction A F B -F
reaction
action

2. action:
rocket pushes
gases backward
re-action:
gases push
rocket forward

3. action:
runner
pushes
ground
backward
re-action:
ground pushes
runner forward

4. F -F Ex. Punching things: Action-reaction forces are ______________ in
Case 1: punching paper
force of paper
on fist:
force of fist
on paper: -F F
Which object exerts a greater force?
neither
Case 2: punching a cement block
force of block
on fist:
force of fist
on block: -F F
Which object exerts a greater force?
neither
Action-reaction forces are ______________ in
case 2, but both cases are opposite and _________ .
greater
equal

5.  Forces ALWAYS occur in __________ .
 Either force could be called the ___________.
 One force does not happen _____________
 Both forces occur at the ___________________.
pairs
action
“first.”
same time
A/ Both forces are the exact same ___________ .
type
Ex 1: F: Earth ________ you down with _________.
-F: You _______ Earth up with _________.
pulls
gravity
pull
gravity
Notice the "same types:" Fg
Both forces are ____________ .
Both forces are ____________
pulls Fg
gravity

6. F: You ________ right on wall with a __________________.
Ex 2:
F: You ________ right on wall
with a __________________.
-F: Wall __________ left on you
wall
push
normal force
hand
pushes
normal force. FN FN
Ex 3:
F: Wire ________ up on picture
with a _______________.
-F: Picture _________ down on wire
with a________________.
pulls
tension
pulls T
tension T

7. B/ Action:reaction pairs act on ______________
different
B/ Action:reaction pairs act on ______________
objects in _____________ directions.
 Action: A exerts a F on B.
 Reaction: ___ exerts a -F on ____.
opposite B A
subject
object
direction
Ex 1: F: Earth pulls you down with gravity.
-F: ______ pull _______ ____ with gravity.
You
Earth up
opposite
____________
direction
subject
object
Notice how subject and object are _____________ ,
and the directions are ________________ .
reversed
opposite

8. Ex 2:
F: You push right on wall with a normal force.
-F: ______ pushes ______ on ______ with a
normal force.
left
Wall
you
Ex 3:
F: Wire pulls up on picture with a tension.
-F: __________ pulls _________ on ________with
a tension.
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
object
direction

9. in ___________ direction on ___ (object) with _____________force." B
Action: Reaction:
"A (subject)
pulls/pushes
in a certain direction on
B (object)
with a certain force."
" ____ (subject)
_________________
in ___________ direction on
___ (object)
with _____________force." B
pulls/pushes
opposite A
the same
Which force, action or reaction, has a greater
magnitude?
neither

10. Ex: Write the action (A) and reaction (R) pairs.
block
table
Case 1/ Between block and table: A: R:
Block pushes down on table with normal force.
Table pushes up on block with normal force.
Case 2/ Between block and Earth: A: R:
Earth pulls down on block with gravitational force.
Block pulls up on Earth with gravitational force.
Which of these are contact forces?
Which are "at a distance" forces?
case 1
case 2

11. Ex: Weight is the force of Earth’s
___________ pulling ______________
on an object:
w = ____ = Earth pulls__________
on cat with a
________________ force.
gravity
downward w Fg
downward w
gravitational w
What is the reaction force to a
cat's weight, w? w
The _______ _______ _______
on ________ with a
__________________ force.”
cat
pulls up w
Earth
gravitational

12. m forces Ex. Don’t confuse ___________ (which are always
equal and opposite) with ___________________
(which depend on _________ and usually are NOT
equal and opposite).
accelerations
mass
Ex: An apple and the Earth (not to scale) F
gravitational F of
Earth on ________
apple: a =
9.81 m s2 =
apple m
gravitational F
of apple on _________ F
Earth
Earth: a = m ≈
m's
Same _____ but different _____ b/c different _____ !
F's
a's