The document discusses Newton's Third Law and how it applies to examples of action-reaction force pairs such as a hammer and nail. It then covers the concepts of momentum, calculating momentum, conservation of momentum, and elastic versus inelastic collisions. Examples are provided to illustrate these physics principles around forces, momentum, and collisions.

1. Section 12.3
NEWTON’S THIRD LAW

2. Newton’s Third Law
 When an object exerts
a force on a second
object, the second
object exerts an equal
and opposite force on
the first
 Action/reaction forces

3. Examples of Action/Reaction
Forces
 Hammer and nail
 Swimming
 Do they always produce
motion?
 Do they cancel and
produce a net force of 0?

4. Think About It…
 Are brakes on a
bicycle or a car
larger?
 Is it easier to catch a
bowling ball or a
tennis ball?
 Without a glove, is it
easier to catch a
baseball thrown
lightly or forcefully?

5. Momentum
 Product of an object’s
mass and velocity
 Momentum (p) = m × v
 Can vary based on your
frame of reference

6. Calculating Momentum
 What is the momentum
of a 7-kilogram bowling
ball that’s heading
towards the pins at 6
m/s?
 What are the units for
momentum?

7. Calculating Momentum
 What is the
momentum of a
200,000-kilogram
train traveling into
the station at 18 m/s?
 Is the momentum of
a race car (9000 kg)
going at 95 m/s
higher or lower than
the train?

8. Calculating Momentum
 A man on a bicycle (total
mass of 85 kg) rides
down the street at 9 m/s.
 A woman in a truck
backs slowly out of her
driveway at 2 m/s. The
truck’s mass is 950 kg.
 Which has a higher
momentum?

9. Momentum is a vector!
 Momentum can be
positive or
negative, depending
on the direction of
motion?
 YOU NEED TO DRAW
A PICTURE TO
EXPLAIN THE
DIRECTIONS!

10. What is the total momentum of
the objects?
 Ptotal = m1v1 + m2v2
 Two kids in two bumper cars
travel towards one another.
The first (mass of 80 kg)
travels at 5 m/s while the
other (mass of 75 kg) travels
towards the first at 8 m/s.

11. What is the total momentum of
the system?
 A pitcher throws a
baseball towards the
plate at 42 m/s. The
baseball has a mass of
0.25 kg. The batter
swings his bat (which
has a mass of 0.90 kg)
towards the ball at 25
m/s. What is the total
momentum?

12. Rolling Wagon
 A child with a mass of
42 kg is rolling down a
hill in a wagon. If the
speed of the wagon is
7.5 m/s and the mass of
the wagon is 30
kg, what is the total
momentum of the
system?

13. Conservation of Momentum
 What does it mean for something to be “conserved”?
 What happens when two bowling balls hit one another
at the same speed but traveling in opposite directions?
 If a moving object hits a non-moving one and they stick
together, will the two objects be moving at a slower
speed or a faster speed?

14. Conservation of Momentum
 Momentum of a
system is equal before
and after a collision
(as long as no outside
forces, like friction, are
acting on the objects of
the system)

15. Conservation of Momentum
 pbefore = pafter
 mbeforevbefore = maftervafter
 You might have to
calculate the TOTAL
momentum before/after
the collision if there are
more than two objects.

16. Types of Collisions
 Elastic Collision  Inelastic Collision

17. Example #1
 In billiards, a cue ball is hit and
begins rolling across the table at 3.5
m/s. It then strikes another ball.
The cue ball stops dead. If the balls
are of equal weight/mass, how fast
does the other ball travel after the
collision?

18. Example #2
 A man pushes a 2 kg model train
car along a track so that it reaches
a velocity of 9 m/s. It then collides
with a second 2 kg train car (not
moving) and the two cars stick
together and continue down the
track. At what speed are the
linked-together cars traveling
now?

19. Example #3
 A wheeled 300 kg cannon sits at
rest with a 20 kg cannonball
inside of it. When fired, the
cannonball shoots east at 70
m/s. Which direction will the
cannon travel in and at what
velocity?