Newton's third law

1. Newton’s 3rd
Law of Motion
8TH GRADE SCIENCE

2. Newton’s 3rd Law of Motion
• AKA “law of action/reaction”
• States that for every action force
there is an equal but opposite
reaction force
• Forces always occur in pairs,
equal but opposite

3. Example:
• This means that when you push on a wall,
the wall pushes back on you with a force
equal in strength to the force you exerted.
• Either force can be considered the action
force or the reaction force.

4. Equal and
Opposite –
NOT
BALANCED
• Action and
reaction force pairs
don’t cancel
because they act on
different objects.
The critter is
applying an action
force on the wall, the
wall is applying a
reaction force on the
critter.

5. How does it
affect me?
• You constantly use
action-reaction force
pairs as you move
about.
• When you jump, you
push down on the
ground.
• The ground then
pushes up on you. It
is this upward force
that pushes you into
the air

6. What about the world around me?
• When a bee flies, its
wings push in a
downward and a
backward direction.
• By Newton’s third law,
the air pushes back on
the bee in the opposite
directions—upward and
forward.
• This force keeps a bee in
the air and propels it
forward.

7. Mass Matters
Larger Mass
• When you walk forward,
you push backward on the
ground.
• Your shoe pushes Earth
backward, and Earth
pushes your shoe forward.
• Why do you move forward
instead of the Earth
moving back?
Smaller Mass
• Take 30 second to discuss
the following with your
shoulder partner
• What happens if you step
on something that has
less mass than you?
(maybe a skateboard)

8. That’s how
rockets
work?
• When the rocket fuel is
ignited, a hot gas is
produced.
• As the gas molecules
collide with the inside
engine walls, the walls exert
a force that pushes them out
of the bottom of the engine.
• What are the action and
reaction forces in the rocket
launch?

9. •This downward push is the
action force.
•The reaction force is the
upward push on the rocket
engine by the gas molecules.
•This is the thrust that propels
the rocket upward.

10. Product:
• How do action/reaction
forces compare to
balanced forces?
• You have two minutes to
brainstorm on your own
• You have one minute to
discuss with your shoulder
partner
• You have five minutes to
compose your answer in
complete sentences.
• Brainstorm
• Discuss
• Compose