QOD QOD QOD QOD
If you went to space, what
would change?
Your weight?
Or your mass?
Newton’s Third Law Review
Push or Pull
• A force is a push or a pull upon an object
which results from its interaction with
another object.
• Ex:
– When you sit in your chair, your body exerts a
downward force on the chair and the chair
exerts an upward force on your body.
Action - Reaction
• There are two forces resulting from this
interaction — a force on the chair and a
force on your body.
• These two forces are called action and
reaction forces and are the subject of
Newton's third law of motion.
Newton's third law is:
• "For every action, there is an equal and
opposite reaction.“
The statement means that in every
interaction, there is a pair of forces
acting on the two interacting objects.
• Forces always come
in pairs – equal and
opposite actionreaction force pairs.
What are the two forces in:
A fish swimming
• Consider the
propulsion of a fish
through the water. A
fish uses its fins to
push water
backwards. But a
push on the water will
only serve to
accelerate the water.
• In turn, the water
reacts by pushing the
fish forwards,
propelling the fish
through the water.
What are the two forces in:
The flight of a bird.
• Consider the flying motion of
birds. A bird flies by use of its
wings. The wings of a bird
push air downwards. In turn,
the air reacts by pushing the
bird upwards. The size of the
force on the air equals the size
of the force on the bird; the
direction of the force on the air
(downwards) is opposite to the
direction of the force on the
bird (upwards).
What are the two forces in:
Driving a car (wheels on the ground)?
• Consider the motion
of your automobile on
your way to school.
An automobile is
equipped with wheels
that spin backwards.
As the wheels spin
backwards, they push
the road backwards.
In turn, the road
reacts by pushing the
wheels forward.
Questions
• 1. While driving, Anna Litical observed a
bug striking the windshield of her car.
Obviously, a case of Newton's third law of
motion. The bug hit the windshield and the
windshield hit the bug. Which of the two
forces is greater: the force on the bug or
the force on the windshield?
• The baseball forces
the bat to the right (an
action); the bat forces
the ball to the left (the
reaction).
• Note that the nouns in
the sentence
describing the action
force switch places
when describing the
reaction force.
Action-Reaction Pairs
• According to Newton's third law, for every
action force there is an equal (in size) and
opposite (in direction) reaction force.
Forces always come in pairs — known as
"action-reaction force pairs."
The action force is stated; determine the reaction
force. Use the pop-up menu to view the answer.
• Athlete pushes bar
upwards.
• Bowling ball pushes
pin rightwards.
• A. The pin pushes the
bowling ball leftwards
• 2. Rockets are unable to accelerate in
space because ...
• there is no air in space for the rockets to
push off of.
• there is no gravity is in space.
• there is no air resistance in space.
• ... nonsense! Rockets do accelerate in
space. How?
Jet pack!
• Identify at least five pairs of action-reaction
forces in the following diagram.