Transcript Slide 1

Newton’s
rd
3
Law
Newton’s 3rd Law
• Forces always exist in
pairs
Examples of Newton’s 3rd
Law
• When you
walk across
the floor:
you push
against the
floor and the
floor pushes
against you
For Every Action There is
an Equal and Opposite
Reaction
• When two objects
interact with one another,
the forces they exert on
each other are called
action-reaction pairs
Action-Reaction Pairs
• Whenever a force is exerted,
there is another force that is
equal in size and opposite in
direction
• This is true even if there is no
motion
Person Pushing Against a
Wall
Force pairs do not always
act on the same object
• Even though the forces are
equal and opposite, they do not
cancel each other out because
they act on different objects
Swimming
• You push the water
backward and the water
pushes you forward
Hammer & Nail
• Nail exerts a force
on the hammer
equal and
opposite the force
the hammer
exerts on the nail
• Net force acting
on the nail, drives
it into the wood
Rocket at Lift Off
• Action:
rocket
pushes on
gas
• Reaction:
gas pushes
on rocket
Equal Forces don’t always
have equal effects
• Because of Newton’s second
law (F=ma) , an object’s mass
will have an impact on the
acceleration from the force
For Example
• When a gun is fired,
the force exerted on
the bullet is as great
as the reaction force
exerted on the gun.
Why is the recoil not
at the same speed as
the bullet?
These forces depend on
friction
• A person or
car on ice may
not be able to
exert the
action force to
produce the
needed
reaction force