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Newton’s Third Law of Motion
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Newton’s Third Law of Motion
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Newton’s first law of motion is
descriptive and his second law is
mathematical
In both cases, we consider the forces
acting on only one object
However, when your hand pushes on a
desk in one direction, you feel a force
of the desk pushing back on your hand
in the opposite direction
This brings us to the third law, which
considers forces acting in pairs on two
objects
Newton’s Third Law of Motion
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To illustrate Newton’s third law of motion, imagine a ball
shot horizontally out of the tube of a toy car on wheels
When the ball is pushed into the card, a spring becomes
compressed
Then, when the spring is released, the spring (and thus
the cart) pushes forward on the ball
We call this force, the action force
Newton’s Third Law of Motion
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At the same instant, the ball pushes backwards on the
spring (and thus the cart)
We call this the reaction force of the ball on the cart
The action and reaction forces are equal in size but
opposite in direction, and act on different objects
Newton’s Third Law of Motion
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Newton’s third law of motion deals with these action
and reaction forces
For this reason, it is sometimes called the action-reaction
law
Newton’s Third Law of Motion (copy)
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If object A exerts a force on object B, then object B
exerts a force equal in size, but opposite in direction
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For every action force, there is a reaction force equal in
magnitude but opposite in direction
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There is no situation where only a single force acts
A single force is impossible……there is always a
corresponding reaction force
Why do they not cancel each other out? This is because
they are acting on different bodies
Eg. A skater bumps into a board
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The skater is exerting a force on the board (action force)
The board is exerting a force on the skater (reaction force)
(copy) – add to note
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The action and reaction forces act on different objects
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***since the action and reaction forces are acting on
different objects, they will appear on separate FBDs
Practice/Examples
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1. Describe the reaction force for each of the following
forces
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A) the southward force of a field goal kicker’s toe on a football
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the northward force of the football on the kicker\s toe
B) the backward force of a jogger’s shoe on the ground
The forward force of the fground on the jogger\s shoe
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C) the downward force of a book on a desk
The upward force of the desk on the book
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2. Two skates are standing on ice facing each other.
Skater 1 pushes on skater 2 with a force of 70 N[E]. No
friction acts on either skater. The mass of skater 1 is 50
kg and the mass of skater 2 is 70 kg.
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A) state the action and reaction forces
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B) state what will happen to each skater
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C) Draw the FBD of each skater
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D) calculate the acceleration of each skater
Newton’s Third Law - Applications
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Newton’s third law of motion has many interesting
applications…
Newton’s Third Law -- Tides
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One of the most striking action-reaction pairs is fond in
the Earth-moon system
The moon attract the water at A more strongly that is
attract the Earth at B because the water is closer to the
moon than the Earth
As the water is free to move, its level is raised on the side
closest to the moon and a high tide is created
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Similarly, the moon attracts the Earth at B more strongly
than the water at C since this water is on the side of the
Earth opposite the moon
Due to inertial effects, the water at C in a sense is “left
behind” because there is less force on it than on an
equivalent mass of Earth
Consequently, a high tide results at C
Practice
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Worksheet (questions #1-4)
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Questions on page 141, #4, 5, 6, 7, 9
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Time for a QUIZ (Forces and Newton’s Laws)
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 try the practice one on my website