1. Momentum and Impulse Conservation of Momentum Collisions
Lec 05: Linear Momentum
Momentum and Impulse
Conservation of Momentum
Collisions

2. Momentum Momentum = (mass ) (velocity) P = m v
Momentum has a magnitude and direction (vector)
Example: A 20,000-kg train is moving at 22 m/s (about 50 mph). How much is its momentum?

3. Force-Momentum Relation
Forces must act for a certain amount of time in order to change the momentum of an object
Change of momentum = (force) (time)
Question: You jump from a tall tree. Which hurts less: keeping your knees stiff or bending them during landing?

4. Puzzles
P1. A ball moving at 5 m/s hits a wall as it stops momentarily. How much is the change in its momentum?
P2. A ball moving at 5 m/s hits and bounces off a wall at 3 m/s. How much is the change in its momentum?
P3. A ball moving at 5 m/s hits a wall and breaks. It emerges behind it with velocity of 3 m/s. How much is the change in its momentum?
P4. You hit a board with an equal effort. Which hurts less: if the board breaks or if it doesn’t?

5. Superman saves Lois Lane, or does he?
The Big Bang Theory Season 1 Episode 1

6. Conservation of Momentum
When two objects interact with each other, they cannot change their overall momentum
(mass1) x (velocity1 ) = (mass2) x (velocity2)

7. “Collisions”
Example: You (50 kg) push your friend (40 kg) on a frictionless ice. If your friend begins to move at
2.0 m/s, how much will be your speed?
Example: Two bugs collide in midair. If a 20-g bug flying north at 2 m/s collides with a 10-g bug flying at 1 m/s south and both stick together, how fast will they be moving after the collision? In which direction

8. “Explosions”
Example: You (50 kg) push your friend (40 kg) on a frictionless ice. If your friend begins to move at
2.0 m/s, how much will be your speed?

9. Puzzles
P1. Why is it difficult for a firefighter to hold a hose that ejects large amounts of water at a high speed?
P2. A person can survive a feet-first impact at a speed of about 12 m/s on concrete; 15 m/s on soil, an 34 m/s on water. Why the different values for different surfaces?
P3. Can a bungee jumper use a steel cable rather than an elastic cord?
P4. You hit boards with an equal effort. Which situation hurts less: if the board breaks or if it doesn’t?