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Physics Teacher Notes - Grothaus
Chapter 7 – Newton’s Third Law of Motion
Title: Newton’s Third Law of Motion – Action and Reaction
*** For every force, there is an equal and opposite force ***
Can’t touch something without it touching back, can’t exert a force
on something without it exerting a force back (can you clap
with only one hand?)
Demonstrations: wall, spring scales, bathroom scales, balloon
Kayak paddles push back on the water, but the water pushes
back which is why the kayak goes forward
7.1
Forces and Interactions
A Force is a push or a pull – this is the simple explanation
Not a true indication of what is happening:
A force is always part of a mutual action that involves
another force
A mutual action is an interaction between one thing and
another
Example: hammer and nail, phone book friction – force
indicator
Question: Does a stick of dynamite contain force?
7.2
Newton’s Third Law
Newton’s third law states that whenever one object exerts a
force on a second object, the second object exerts an
equal and opposite force on the first object.
(“To every action there is always an equal and opposing
reaction”)
One force is called the action force and the other is called the
reaction force
Example: swimming, walking across the floor vs. walking on
ice, dog wagging his tail
7.3
Identifying Action and Reaction
Conceptual Physics
Page 1 of 3
Chapter 7 – Newton’s Third Law of Motion
Physics Teacher Notes - Grothaus
What is the pair of action and reaction forces in the case of a
falling boulder?
Not always obvious what these are.
To identify a pair of action-reaction forces, first identify
the interacting objects A and B, and if the action is A
on B, then the reaction is B on A.
In the above pair, the interaction is the gravitational attraction
between the boulder and the Earth.
Action: Earth exerting force on boulder
Reaction: Boulder exerting a force on Earth
Why doesn’t the Earth move up towards the
boulder?
Is this true? Reaction forces occur slightly after the action
force is applied.
NO!! They occur simultaneously
See page 109, figure 7.5
https://www.youtube.com/watch?v=jHbyQ_AQP8c
“What if we all jumped at once?”
7.4
Action and Reaction on Different Masses
Look at the interaction between a falling boulder and the Earth.
The boulder pulls up the Earth with as much force as the
Earth pulls down on the boulder.
The masses are very different. Newton’s second law states
𝑎
that 𝑎 =
. Both Earth and the boulder accelerate!
/𝑎
Use See-Saw to look at what happens!
Force and Mass
Firing a cannon: Figure 7.7 p. 110
The cannon “kicks”
Conceptual Physics
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Physics Teacher Notes - Grothaus
Cannonball:
Chapter 7 – Newton’s Third Law of Motion
Cannon:
A given force exerted on a small mass produces a
greater acceleration than the same force exerted on
a large mass.
Cannon vs. Cannonball explains rocket propulsion.
Balloon: air escapes – balloon accelerates
Rocket accelerates much the same way.
It recoils from the exhaust gases ejected from its engine
Each molecule acts like a tiny cannonball
Common misconception: rocket is propelled by the
impact of the gases against the atmosphere.
Better above atmosphere – no air resistance
Lift
Helicopter’s rotors are shaped to force air particles downward
(action) and the air forces the blades upward (reaction)
Same with an airplane: the slightly tilted wings and jet
propulsion
7.7
Action Equals Reaction - doing
You can only hit something as hard as it can hit you back.
Wall, paper, etc.
Conceptual Physics
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