Notes for Ms. Parker's lab and class Dec 13 & 14, 2011

1. Good Morning!
• Today we will:
– conduct an investigation
– take some notes
– solve some momentum problems
• Please do before the bell:
– get out your lab notebook and pen/pencil
– get a whiteboard and dry erase marker
– open your textbook to pg 310

2. Conservation of Momentum

3. Warm-Up
• On your whiteboard, solve the following
problem. SHOW GUTS!
• What is the momentum of a 0.3 kg cart
moving at a velocity of 0.5 m/s?

4. Lab: Conservation of Momentum
• Open your lab book to the nearest clean page
and title this lab, “Conservation of
Momentum.”
• Be sure to make an entry into your Table of
Contents as well.

5. Lab: Conservation of Momentum
• Groups of 5 – 6
• Everyone writes down all data and answers in
their lab book DURING the lab
• We will do Steps #1 – 3 in our lab
• Step #4 will be part of your exit slip at the end
of class

6. Conservation of Momentum
• Momentum, like energy, is conserved.
• What does this mean for a car crash?
– In a car crash, if you add up all the momentum
before a collision, it will equal the total
momentum after a collision.
• Momentum BEFORE collision = Momentum AFTER collision

7. Conservation of Momentum
• Of course, there is a formula
• m1v1 + m2v2 = (m1 + m2)vf
• Why do we add the masses together after the
collision?
– because they stuck together and travel at the
same speed

8. Conservation of Momentum
• The conservation of momentum is true
whether or not the objects “stick”
together, however so the better formula might
be:
• m1v1b + m2v2b = m1v1a + m2v2a
• Momentum Before = Momentum After

9. Law of Conservation of Momentum
• The total momentum before a collision is
equal to the total momentum after the
collision if no external forces act on the
system.
– this is true for all collisions between any objects!
• a car and a truck
• two railroad cars
• a proton and a proton
• a planet and a meteor

10. Conservation of Momentum
• Conservation of momentum is a crucial piece
to understanding and predicting motion.
• Since we know the total momentum in the
beginning of a collision, we know the total
momentum after the collision.
• We can use this understanding to help us
predict the results of any collision.

11. Sample Problem #1
• A 75 kg boy and a 50 kg girl are riding in
identical bumper cars at an amusement park.
The boy’s car is moving to the east at 3.00 m/s
and the girl is moving west at a velocity of
1.80 m/s. If they stick together on
collision, what is their final velocity?

12. Conservation of Momentum in Elastic
Collisions
• What if the two objects don’t stick
together, but move away in the same direction
at different speeds?
• What formula should you use?
–m1v1b + m2v2b = m1v1a + m2v2a

13. Sample Problem #2
• A steel ball with a mass of 2kg is traveling 3 m/s
west. It collides with a stationary ball that has a
mass of 1 kg. Upon collision, the smaller ball
moves to the west at 4 m/s. What is the velocity
of the larger ball?

14. Checking In
• A vehicle with 6000 kgm/s of momentum
collides with a car at rest. As they slide off
together, what is their momentum?