1. Collision

2. Point of Contact  When two objects collide there is a point of contact.  The moment of contact is short. Impulse due to external forces is smallImpulse due to external forces is small J ext = F ext  tJ ext = F ext  t  The objects in collision at the time of collision can be viewed as an isolated system.

3. Momentum at Collision  For an isolated system momentum is conserved.  Always true in collisions. Reaction forces balanceReaction forces balance No time for external forcesNo time for external forces Before: m1m1 v1iv1i v2iv2i m2m2 v1fv1f v2fv2f After:

4. Momentum and Kinetic Energy  An object in motion has a momentum based on its mass and velocity. p = mvp = mv  The object also has kinetic energy. K = ½ mv 2K = ½ mv 2  Kinetic energy is related to the momentum. K = ½ p 2 /mK = ½ p 2 /m

5. Kinetic Energy at Collision  Energy is conserved only for conservative forces.  Internal forces may be nonconservative.  The force at the collision is not always conservative. energy lost to heat Before: m1m1 v1iv1i v2iv2i m2m2 After: v2fv2f v1fv1f

6. Elastic Collision  For conservative forces the energy is conserved.  After the collision of contact the potential energy is zero.  The total kinetic energy is conserved – equal before and after the collision.  This an elastic collision. Elastic

7. Inelastic Collision  For non-conservative forces some energy is lost.  After the collision there is less energy available.  The total kinetic energy is not conserved – less after the collision.  This is an inelastic collision. next Inelastic