Download Newton`s 3rd Law

Essential Question
• What is Newton’s Third Law
& how does it apply to
motion?
Newton’s First Law
“An object at rest tends to stay at rest
and an object in motion tends to stay
in motion unless acted upon by an
unbalanced force.”
Newton’s Second Law
“Force equals mass times acceleration.”
F = ma
Acceleration: a measurement of how quickly an
object is changing speed.
More Laws of Motion
• Newton’s First Law of Motion states that if
you kick a ball, the ball will move.
• Newton’s Second Law of Motion says that
when a force acts on an object, the object
accelerates.
– If you kick the ball harder, it will move faster.
– It also tells you that a heavy ball is harder to
move than a lighter ball.
Newton’s Third Law
“For every action there is an equal and
opposite reaction.”
OUCH!
• Let’s say you were running and weren’t
paying attention where you were going
and ran into a wall. Are you hurt because
you hit the wall?
• No. You can’t feel the forces on the wall.
You DO feel the force on your body!
You’re hurt because the wall hit you!
Forces occur in
pairs
As the runner’s foot
pushes on the
starting block
(action force). The
starting block is
exerting a force on
the runner’s foot
(reaction force).
A Horse of a Good Time
• A horse pulls on a cart. Can you name the
opposing force?
• Think of it this way…the horse pulls on the
cart. So Newton’s Third Law tells you that
the cart pulls the horse.
The cart pulls the horse
The horse pulls the cart
What does this mean?
For every force acting on an object, there is an equal
force acting in the opposite direction. Right now,
gravity is pulling you down in your seat, but
Newton’s Third Law says your seat is pushing up
against you with equal force.
2 Forces at Work
• The forces that 2 objects put on each
other are called an action-reaction force
pair.
• The forces in the force pair are equal in
strength, but opposite in direction.
• One is called the action force.
• The other is called the reaction force.
Newton’s 3rd Law at work…
When a rocket blows out gas at high
speeds in one direction (action force),
The rocket is pushed in the opposite
direction (reaction force).
The gas pushes against the rocket
and the rocket pushes back just as
hard against the gas.
This is known as thrust!
More examples…
Discuss with your lab partner which forces are action and reaction
forces.
Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push against
a wall? You slide in the opposite direction
(away from the wall), because you pushed on
the wall but the wall pushed back on you with
equal and opposite force.
Why does it hurt so much when you stub
your toe? When your toe exerts a force on a
rock, the rock exerts an equal force back on
your toe. The harder you hit your toe against
it, the more force the rock exerts back on your
toe (and the more your toe hurts).
Review
Newton’s First Law:
Objects in motion tend to stay in motion
and objects at rest tend to stay at rest
unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration
(F = ma).
Newton’s Third Law:
For every action there is an equal and
opposite reaction.
National Science Foundation
• http://www.nsf.gov/news/special_reports/fo
otball/newtonthirdlaw.jsp
Let’s Review
• Newton's third law of motion states that __________.
A) an object will remain at rest or keep moving in a straight
line with constant speed unless a force acts on it
B) when a force is applied on an object, there is an equal
force applied by the object in the opposite direction
C) acceleration is calculated by dividing the force exerted
on an object by the mass of the object, and that when a
force acts on an object, its acceleration is in the same
direction as the force
D) the force on an object can be found by dividing its mass
by its acceleration
Let’s Review
• Whenever one object exerts a force on
another object, the second object
ALWAYS exerts an equal force back on
the first object.
– A.) True
– B.) False
Let’s Review
• Forces always occur in ______________.
– A.) Triplets
– B.) Pairs
– C.) by themselves
– D.) either singly or in pairs
Let’s Review
• The two forces of a force pair are called
the __________ force and the
__________ force.
•
•
•
•
A) mass, acceleration
B) gravitational, accelerator
C) increasing, decreasing
D) action, reaction
Common Misconception…
• http://www.youtube.com/watch?v=8bTdM
mNZm2M