Download Newton`s 3rd Law

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
• If the mass is
more
• When an object
said to be
, the object will
, the object will
.
.
, it is also
. Huh? You will see…
• 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.
What is the
of a
• If you pull on a rope, do you feel the rope
pull on you?
• Which is the source of the force, you or the
rope?
?
• All forces exist in
(no
) that are
in
equal in magnitude (size).
and
• When you kick a ball, you exert a force on the
while the
exerts a force on you.
• You pull on a to open it while the
exerts
a force on you.
• A
exerts a force on a nail while the
exerts a force on the hammer.
• When you
on a rope, it
on you.
Newton’s
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
there is an
. (Newton’s ____
Law is sometimes referred to as the “
of
.”)
Free Body Diagram of Newton’s 3rd
Law of Motion
=
Note that the forces come in
that have
.
Action - Reaction
-------Reaction
Action
=
=
=
Note: All forces come in
and
pairs
What is the
Force?
• You push on your desk.
• The
on you.
• An airplane propeller pushes on the air.
• The
on the airplane propeller.
• A tennis racquet hits a ball.
• A
a tennis racquet.
• You push on the floor as you take a step.
• The
back on you.
Do the forces cancel each other
out?
•
.
• Since the forces act on
possible for them to
Ff = mfaf
Force of floor on
person.
, it is
each other out.
Fp = mpap
Force of person
on floor.
rd
3
Newton’s
Law of Motion &
Acceleration
• Newton’s 3rd Law of Motion says that
____ = ____
• And Newton’s 2nd Law of Motion says that
______ = ______
• While the forces may be
direction, the
but opposite in
of each object may be
.
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?
•
, because both forces act on the
=
object.
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
to the
of the force
simultaneously exerted by object 2 on object 1, and are in
directions.
• Newton’s 3rd Law of Motion is also known as the
of
.
• All forces in nature come in
.
• While the forces are equal and opposite, they do
each other out because the two forces act on
objects.