Conservation of Momentum Notes

1. CP Physics Conservation of Momentum Notes

2. Law of Conservation of Momentum • In closed, isolated system – total momentum remains constant • Special case of Newton’s 3rd Law • Closed system = nothing enters or leaves • Isolated = no net external forces act on system • All changes within system cancel each other out so total momentum stays same

3. Example of Conservation of Momentum • Bus traveling down highway struck by bug which splatters on windshield • Same force acts on both bug and bus • Time of impact same for both • Both experience same impulse • Both experience same change in momentum (impulse) • Bug – greater change in velocity because mass much smaller • Bus – no noticeable change in velocity because mass huge compared to bug • Bug suffers much greater damage than bus

4. Two Types of Collisions • Elastic • Total KE remains same before and after collision • Objects involved in collision “bounce” off each other • Inelastic • Some KE is lost as heat • Objects involved in collision become stuck together or become deformed

5. Question 1 • A spacecraft in outer space increases velocity by firing its rockets. How can hot gases escaping from its rockets change the velocity of the craft when there is nothing in space for the gases to push against? • The gases push against the spacecraft and the spacecraft pushes back so the spacecraft and gases move in opposite directions.

6. Question 2 • The white cue ball travels across a pool table and collides with the stationary eight ball. The two balls have equal mass. After the collision the cue ball is at rest. What must be true of the speed of the eight ball? • All momentum of cue ball transferred to eight ball and since they have same mass, the eight ball will move at same velocity as the cue ball.

7. Question 3 • Two trucks that look the same collide. One was originally at rest. The trucks stick together and move off at more than half the original speed of the moving truck. What can you say about the contents of the two trucks? • Moving truck is more massive than the other truck. If had same mass, velocity would have become half when their mass doubled. Since velocity is more than half the original velocity, the stationary truck must have less mass than the moving truck.

8. Steps to Solving Conservation of Momentum Problems • Read problem. • Identify objects’ masses and velocities before and after collision. • Remember – if objects become stuck together – add their masses after collision. • Use x to represent missing velocity. • Calculate objects’ momentums before and after collision. • Set up momentums into algebra equation with equal sign separating before and after collision. • Solve for x.