Momentum and Impulse Conservation of Momentum Collisions Lec 05: Linear Momentum Momentum and Impulse Conservation of Momentum Collisions
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?
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?
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?
Superman saves Lois Lane, or does he? The Big Bang Theory Season 1 Episode 1
Conservation of Momentum When two objects interact with each other, they cannot change their overall momentum (mass1) x (velocity1 ) = (mass2) x (velocity2)
“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
“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?
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?