1. SCI 340 L21 conservation of momentum
Group Work
A hockey puck of mass 0.25 kg slides eastward across the ice at 25 m/s.
What is its momentum p1 (magnitude and direction)?
The puck collides with a hockey stick that was lying motionless on the ice while its owner fights. The puck rebounds in the exact opposite direction of its approach, moving at a speed of 10 m/s. Now what is its momentum p2 (magnitude and direction)?
What was the momentum change Dp = p2 – p1 of the puck from before to after the collision (magnitude and direction)?
Props: “Give a Little Bit” song files
Notes: Write quantities on the board and keep them there throughout the lecture for later reference

2. Question
When a bug hits a car windshield, which one experiences the greatest force?
The bug.
The car.
It’s a tie.
Need more information to know.

3. Newton’s third Law
Remember it

4. Question
When a bug hits a car windshield, which one experiences the greatest impulse?
The bug.
The car.
It’s a tie.
Need more information to know.

5. Question
When a bug hits a car windshield, whose momentum changes the most? (Assume there are no external forces.)
The bug’s.
The car’s.
It’s a tie.
Need more information to know.

6. All-Class Work
Show that when two otherwise isolated objects interact, their total change in momentum is zero.
Dp1 + Dp2 = 0 or
Dp1 = –Dp2
Hint: When force F1 is applied to the first object for time Dt, what is its momentum change? What happens to the second object during this time?

7. Conservation of Momentum
This is important
§ 7.2

8. Conservation of Momentum
The total momentum of an isolated system never changes.

9. Conservation of Momentum
Newton’s first law:
no outside force
no change in v
thus no change in p
So an isolated object’s momentum never changes.

10. Conservation of Momentum
Newton’s third law:
interacting objects apply equal and opposite impulses to each other
they experience equal and opposite momentum changes
So their total momentum remains the same.

11. Group Work
Continue the hockey scenario from problem 1. (puck Dp = 8.75 kg m/s W)
What was the momentum change of the hockey stick during the collision? (Momentum is conserved.)
What is the momentum of the stick after the collision?
If the hockey stick has a mass of 1.0 kg. What is its velocity (magnitude and direction) after the collision?

12. Collisions When objects meet and interact
Total momentum is always conserved

13. SCI 340 L21 conservation of momentum
Elastic Collisions
Objects bounce apart after collision
same relative speeds as before
Total momentum is conserved
Some momentum is transferred from one object to another
Kinetic energy is also conserved

14. Totally Inelastic Collisions
Objects cling together after collision
same final velocity
Total momentum is conserved in the coupled mass
Kinetic energy is not conserved

15. SCI 340 L21 conservation of momentum
Inelastic Collisions
Objects bounce apart after collision
relative speed less than initial
Total momentum is conserved
Kinetic energy less than initial (not conserved)

16. Group Work
If, instead of bouncing apart, the puck and stick clung together when they collided (totally inelastic collision), what would their velocity (magnitude and direction) be after the collision?
Hint: What is their momentum after the collision?

17. Group Work
Don’t calculate, just think and answer: If the puck were initially moving eastward, but had rebounded off the stick so that it moved northward after the collision, which direction would the stick have been moving after the collision?
Stick and puck have opposite momentum changes.