1. Elastic Collisions

2. Conservation  Elastic collisions conserve both momentum and kinetic energy.  Two equations govern all elastic collisions. m1m1 m2m2 v1iv1i v2iv2i before m1m1 m2m2 v1fv1f v2fv2f after

3. Head-on Collision  An elastic head-on collision takes place in one dimension.  If the collision is not head- on, the force pair is in a different direction. m1m1 m2m2 v1iv1i v2iv2i m1m1 m2m2 v1iv1i v2iv2i force and velocity in a lineforce and velocity on different lines

4. Related Velocities m1m1 m2m2 v1iv1i v2iv2i momentum in a line kinetic energy conservation solve for velocities

5. Equal Masses  A 150 g ball moves at 1.4 m/s. The momentum is 0.21 kg m/sThe momentum is 0.21 kg m/s  It strikes an equal mass ball at rest. v 1i = 1.4 m/sv 1i = 1.4 m/s v 2i = 0v 2i = 0 Therefore, v 1f = 0Therefore, v 1f = 0 and v 2f = v 1iand v 2f = v 1i m1m1 m2m2 v1iv1i m1m1 m2m2 v2fv2f momentum: kinetic energy:

6. Striking a Heavy Mass  Let m 1 << m 2, when a golf ball bounces off the floor.  The floor is at rest. v 2i = 0v 2i = 0  The final velocity is equal and opposite the initial velocity momentum: kinetic energy: combined: m1m1 v1iv1i v1fv1f

7. Striking a Light Mass  Let m 1 >> m 2, when a golf club strikes a ball.  The ball is at rest. v 2i = 0v 2i = 0  For a very heavy m 1, the final velocity of m 2 is twice the initial velocity of m 1. momentum: kinetic energy: combined: v1iv1i v2fv2f m2m2

8. Impact Parameter  When collisions are not head on, the objects can come out at an angle.  The masses come out at equal angles.  The distance of the center of the ball to the center of line between them is the impact parameter. next m1m1 m2m2 v1iv1i v2iv2i m1m1 m2m2 v1fv1f v2fv2f