rd
3
SECTION 3: Newton’s
Momentum
Law and
LEARNING GOALS
 State Newton’s
of Motion.
 Identify action and reaction forces.
 Calculate momentum.
 Recognize when momentum is
conserved.
rd
3 Law
NEWTON’S
RD
3
LAW
 Newton’s
Law of Motion:
When one object exerts a
force on a second object, the
second object exerts a force
on the first that is equal in
strength and opposite in
direction.
rd
3
ACTION AND REACTION
 Another way of
rd
stating Newton’s 3
Law is “to every
action force there is
an equal and opposite
reaction force”.
ACTION AND REACTION FORCES
DON’T CANCEL!
 If the action and reaction forces
occurred on the same object, they
would result in zero motion.
 The action and reaction forces must
occur on different objects.
ACTION AND REACTION FORCES
DON’T CANCEL!
Example: A swimmer
pushes the water. She
“acts” on the water.
The water “reacts” by
moving and pushing
her forward.
MOMENTUM
 Momentum: property of a moving
object that equals its mass times its
velocity.
 Momentum is related to how much
force is needed to change an object’s
motion.
MOMENTUM EQUATION
p = mv
p = momentum (kg x m/s)
m = mass (kg)
v = velocity (m/s)
 Just like velocity, momentum
has size and direction!
FORMULA SHEET
p = mv
m = p/v
v = p/m
EXAMPLE
 At the end of a race, a sprinter with a mass of 80
kg has a speed of 10 m/s. What is the sprinter’s
momentum?
FORCE AND CHANGING MOMENTUM
 If you catch a baseball, your hand might sting
(even with a glove). This is because the baseball
exerted a force on your hand when it came to a
stop and its momentum changed.
 This can also be calculated!
CALCULATING THE FORCE USING
MOMENTUM
F = (mvf – mvi) / t
F = force in N
mvf = final momentum in kg x m/s
mvi = intital momentum in kg x m/s
t = time
EXAMPLE OF CALCULATING
FORCE USING MOMENTUM
 What is the force exerted by a catcher’s
glove on a 0.15 kg baseball moving at 35 m/s
that is stopped in 0.02 s?
LAW OF CONSERVATION OF
MOMENTUM
 The momentum of an object does not
change unless its mass, velocity, or both
change.
 Momentum can be transferred from
one object to another.
LAW OF CONSERVATION OF
MOMENTUM
 Law of Conservation of Momentum:
the total momentum of objects before
a collision is equal to the total
momentum of objects after a collision.
 Total momentum is not lost or gained
during a collision
LAW OF CONSERVATION OF
MOMENTUM
LAW OF CONSERVATION OF
MOMENTUM