Transcript Newton`s Third Law of Motion

Newton’s Third Law of
Motion
Chapter 6
Forces and Interaction
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When you push on a wall, it pushes back on you
Most people don’t realize that walls can push on you
every bit as you can push on walls
In its simplest sense, a force is a push or a pull
A closer examination shows that a force is not a
thing in itself, but a part of an interaction between
one thing and another
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You and the wall
Hammer and nail
Newton’s Third Law of Motion
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Whenever one object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first object
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For every action there exists an equal opposing reaction
One force is called the action force, the other is
called the reaction force
They are partners in a single interaction in which
neither force can exist without the other
The two forces are equal in magnitude and
opposite in direction
Forces Exist in Pairs
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When we walk…
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When we drive…
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We push against the floor with our feet
The floor pushes back on our feet
The tires push against the road
The road pushes back on the tires
When we swim…
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We push the water backward
The water pushes us forward
Questions?
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Does a stick of dynamite contain force?
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A motor scooter accelerates along the road.
Strictly speaking, what is the force that
moves the scooter
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Earth and the moon are connected by a
gravitational force. Is Earth pulling on the
moon or is the moon pulling on Earth?
Identifying Action and Reaction Pairs
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Identifying reaction pairs is not always obvious
What are the action and reaction forces in the case
of a falling boulder?
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Earth is pulled up by the boulder with just as much force as
the boulder is pulled down by Earth
We know the boulder falls to Earth
Can we also say that the Earth, in turn, falls to the boulder?
Forces are equal in magnitude and opposite in direction
However, the masses are different
Remember, acceleration is inversely proportional to mass,
so the acceleration of Earth is infinitesimally small
Questions
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We know that Earth pulls on the moon. Does the
moon also pull on Earth?
If so, which pull is stronger?
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What happens when you step off a curb onto a
street?
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Can you identify the action and reaction forces of an
object falling in the vacuum of outer space?
Do Action and Reaction Forces Cancel?
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No
If two forces act on the same body in equal and
opposite directions, they cancel
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Internal to the system
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Two feet kick a football with equal force from opposite sides of
the ball
If one body exerts a force on a second body, the
force is independent from the force exerted back on
it from that body
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External from the system
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One foot kicks a football and the football exerts the same force
back on the foot
Question
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Suppose a friend who hears about Newton’s
third law says that you can’t move a football
by kicking it because the reaction force by the
kicked ball would be equal and opposite to
your kicking force. He says that the net force
would be zero, so no matter how hard you
kick, the ball won’t move.
What would you say to your friend?
You can’t cancel a force on a football with a
force on your foot. Period.