Download Action and Reaction

Newton’s 3rd Law of Motion
Action - Reaction
Chapter 6.3
Newton’s 2nd Law (a = F/m)
• Newton’s second law is responsible for explaining
how objects increase or decrease in speed, or
change direction.
• If the force is increased, the object will accelerate.
• If the mass is increased, the object will accelerate more
slowly.
• When an object changes direction, it is also said to be
accelerating.
• Newton’s 2nd Law can also be used to explain the
forces that exist on stationary objects like your
weight or the repulsive and attractive forces of two
magnets.
Questions
• When two people push each other, why do they
move? What if there was only one person?
• 4 wheel skateboards
• How does a rocket work?
What is the source of a force?
• If you pull on a rope, do you feel the rope
pull on you?
• Which is the source, you or the rope?
Interaction Pairs
• All forces exist in pairs (no isolated forces) that
are opposite in direction and equal in magnitude
(size).
• When you kick a ball, you exert a force on the ball
while the ball exerts a force on you.
• You pull on a door to open it while the door exerts a
force on you.
• A hammer exerts a force on a nail while the nail exerts
a force on the hammer.
• When you pull on a rope, it pulls on you.
Newton’s 3rd Law of Motion
• When one object exerts a force on another
object, the second object exerts an equal but
oppositely directed force on the first object.
• For every action there is an equal but opposite
reaction. (Newton’s 3rd Law is sometimes
referred to as the “law of action and reaction.”)
Free Body Diagram of Newton’s 3rd
Law of Motion
Fg(book on Earth)
=
Fg(Earth on book)
Fg = Force of
gravity (Book on
Earth)
“Reaction”
Fg = Force of
gravity (Earth
on Book
“Action”
Note that the forces come in pairs that have symmetry.
Action - Reaction
Fg (car on Earth)
FN (road on car)
Ff (road on car)
Fg (Earth on car)
-------Ff (car on road)
FN (car on road)
Reaction
Action
Fg (Earth on car)
FN (road on car)
Ff (road on car)
=
=
=
Fg (car on Earth)
FN (car on road)
Ff (car on road)
Note: All forces come in equal and opposite pairs
What is the Reaction Force?
• You push on your desk.
• The desk pushes on you.
• An airplane propeller pushes on the air.
• The air pushes on the airplane propeller.
• A tennis racquet hits a ball.
• A ball hits a tennis racquet.
• You push on the floor as you take a step.
• The floor pushes back on you.
Do the forces cancel each other
out?
• No.
• Since the forces act on different objects, it is
not possible for them to cancel each other out.
Ff = mfaf
Force of floor on
person.
Fp = mpap
Force of person
on floor.
rd
3
Newton’s
Law of Motion &
Acceleration
• Newton’s 3rd Law of Motion says that
F1 = F 2
• And Newton’s 2nd Law of Motion says that
m1a1 = m2a2
• While the forces may be equal but opposite in direction,
the accelerations of each object may be different.
F1
F2
Example: Newton’s
Motion
rd
3
Law of
A physics student is pulling on a rope, which in both cases
causes the scale to read 500 Newtons. The physics student is
pulling
a. with more force when the rope is attached to the wall.
b. with more force when the rope is attached to the elephant.
c. the same force in each case.
Newton’s 3rd Law of Motion?
• Does the following diagram illustrate an
action-reaction pair?
• No, because both forces act on the same object.
FN = Normal
force of Desk on
Book
FN
=
Fg
Fg = Force of
Gravity on Book
Key Ideas
• Newton’s 3rd Law of Motion states that when two objects
interact, the magnitude of the force exerted by object 1 on
object 2 is equal to the magnitude of the force
simultaneously exerted by object 2 on object 1, and are in
opposite directions.
• Newton’s 3rd Law of Motion is also known as the Law of
Action-Reaction.
• All forces in nature come in pairs.
• While the forces are equal and opposite, they do not cancel
each other out because the two forces act on two different
objects.