rd
3
Newton’s
Law and
Momentum
Newton’s 3rd Law
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.
Action and Reaction
• Another way of stating Newton’s 3rd Law is “to
every action force there is an equal and
opposite reaction force”.
• Example – You exert a downward force on a
trampoline. The trampoline exerts an equal
force upward on you.
Action and Reaction Forces Don’t
Cancel!
• Action and reaction forces work on different
objects, so they are not balanced.
Example – A swimmer pushes the water. She
“acts” on the water. The water “reacts” by
moving and pushing her forward.
Momentum
• Momentum – how much force is needed to
change an object’s motion.
• Depends on an object’s mass and velocity
• Momentum is given the symbol p.
Momentum Equation
p = mv
p = momentum in kg * m/s
m = mass in kg
v = velocity in m/s
Just like velocity, momentum has size and
direction!
Example Momentum Problem
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 * m/s
mvi = intital momentum in kg* 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 it’s mass, velocity, or both change.
• Momentum can be transferred from one object
to another.
• Example – A bowling ball traveling down the lane
has momentum. That momentum is transferred
to the pins as the ball strikes the pins.