Physics 105, Fall Term, 2009
Reading: Chapter 6.3-6.4
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1) Momentum of objects within a system are conserved through a collision.
2) Elastic collisions are collisions in which the objects bounce off of each other after colliding.
3) Inelastic collisions are collisions in which the objects stick together after colliding.
The following applet is not flashy, but it shows very simply how the conservation of momentum works in collisions: http://www.physics.upenn.edu/courses/gladney/phys1/lectures/lecture7/momentum_conservation.html
This applet is really self explanatory, just hit the 'Start' button to see a collision with the settings you are at and the 'Reset' button to put the objects back in place. You will see green words and numbers flashing right under the point of collision as the applet runs, these numbers give the total momentum, speeds, and kinetic energy of the collisions.
With the elasticity set to 100% and the masses and speeds set to 1, watch the collision. What do you see?
The masses collide at the same speed and bounce off each other at the same speed.
Is momentum conserved?
Is kinetic energy conserved?
Now change mass 1 to 5 and the speed of mass 1 to 3 and watch the collision. What do you see?
The masses collide with mass 1 moving 3 times faster than mass 2. Mass 2 bounces off mass 1 with a much greater speed than it started with. Mass 1 continues to go in the same direction but is a little slower than before the collision.
Now change mass 1 to 1 and the speed of mass 1 to 0 and watch the collision. What do you see?
Mass 1 starts out still while mass 2 moves at mass 1. After the collision, mass 1 is moving at the same speed mass 2 was moving before the collision and mass 2 is not moving.
Now change the speed of mass 1 back to 1 and change the elasticity to 0% and watch the collision. What do you see?
The masses both move toward each other at the same speed and stop moving as they collide.
Now change the elasticity to 50% and watch to collision. What do you see?
The masses approach each other at the same speed and bounce off of each other at about half of the original speed.
Under what condition is momentum in a collision conserved?
Momentum is always conserved in collisions.
Under what condition is the kinetic energy of a collision conserved?
Kinetic energy is only conserved through a collision when the collision is perfectly elastic.
Sometimes it is forgotten that explosions and fuel propulsion are just special cases of collisions and the Law of Conservation of Energy. Watch the following video: http://www.youtube.com/watch?v=nNyzwnQ2Qe8
In this situation, what is the initial mass?
Mass of water in bottles Mass of man Mass of water in bottles and mass of man combined Mass of water vapor exiting the bottle
What is the initial velocity?
Velocity of water in bottles Velocity of man Velocity of water in bottles and man combined Velocity of water vapor exiting the bottle
What is the final mass?
Mass of water in bottles Mass of man Mass of water in bottles and mass of man combined Mass of air exiting the bottle
What is the final velocity?
Velocity of water in bottles Velocity of man Velocity of water in bottles and man combined Velocity of air exiting the bottle
What type of collision is this?
Elastic Inelastic
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Was there anything that you didn't understand in the reading assignment? What was confusing to you?
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