Newton’s Third Law
Chapter 6
For every action there is an
equal and opposite reaction.
True or False?
What Actions?
•
•
•
•
For every FORCE there is an
equal and opposite FORCE.
Money?
Revenge?
Love?
Hitting?
Always True!
What Does it Mean?
• Misconception:
Actions are always
equal and opposite.
• Truth: Only forces are
equal and opposite.
I push you, you fall
down.
I push you, you push
back on me.
What Does it Mean?
• Misconception:
Forces can be very
large on weak
materials like paper.
I can hit a piece of
paper with a million
Newtons of force.
• Truth: You can only
hit something as hard
as it can hit you.
Paper is weak, so you
can’t hit it very hard.
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What Does it Mean?
• Misconception: There
are special cases
where the law is not
true.
If I sneak up on you,
you won’t hit me
back.
• Truth: There are
apparently no
exceptions to the law.
You hit me back
whether you mean to
or not.
What Does it Mean?
• Misconception: Since • Truth: The forces
the forces are equal
ALWAYS act on
and opposite, they
different objects. Only
cancel each other out.
forces that act on the
same object can
cancel out.
What Does it Mean?
• Misconception:
Because the forces
are the same, the
accelerations are the
same.
• Truth: Acceleration
depends on force
AND mass. The
bigger mass will have
less acceleration.
2
I hit the wall…
• What is the reaction force?
• Which force is bigger?
• Which acceleration is bigger?
Bus hits car…
• What is the reaction force?
• Which force is bigger?
• Which acceleration is bigger?
Horse and Cart
Newton’s Third Law
Explaining
Freefall
• Whenever mass gets bigger, the force of
gravity gets bigger, too.
• “g” is the ratio between force and mass.
• “g” remains constant.
3
• Force is the same for both objects!!
Sgt. Bilko
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