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# Conservation of momentum

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

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### Conservation of momentum

1. 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. 2. Conservation of Momentum
3. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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?