Action-Reaction

2. 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.”

3. Newton’s Second Law
“Force equals mass times acceleration.”
F = ma
Acceleration: a measurement of how quickly
an object is changing speed and direction.

4. 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.

5. Newton’s Third Law
“For every action there is an
equal and opposite reaction.”

6. 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!

7. 3rd Law of Motion
Newton’s third law of motion
states that if one object exerts a
force on another object then the
second object exerts a force of
equal strength in the opposite
direction of the first object.
Simply put, for every action there
is an equal but opposite reaction.

8. When one of these skaters pushes on the other,
both skaters move.
The speeds that the two skaters move depends
on their masses. If they have the same mass,
they will move at the same speed. If one has a
greater mass, that skater will move slower.
Action Force
Reaction force

9. Do Action-Reaction Forces
Cancel?
Balanced forces, which are equal and
opposite, cancel and produce no change in
motion.
Newton’s Third Law refers to forces on two
different objects. The action and reaction
forces can not be added together because
they are acting on different objects.
Force on Wrists
Force on Ball

10. 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).

12. A Horse of a Good Time
The horse pulls the cart
The cart pulls the horse

13. 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.

14. 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.

15. 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!

16. 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).

20. 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.

21. 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

22. 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

23. Let’s Review
Forces always occur in ______________.
◦A.) Triplets
◦B.) Pairs
◦C.) by themselves
◦D.) either singly or in pairs

24. 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

25. 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

26. 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

27. Let’s Review
Forces always occur in ______________.
◦A.) Triplets
◦B.) Pairs
◦C.) by themselves
◦D.) either singly or in pairs

28. 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