Elastic Collisions
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 2 = p 2 / 2mK = ½ mv 2 = p 2 / 2m
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
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
Double 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
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
Related Velocities m1m1 m2m2 v1iv1i v2iv2i momentum in a line kinetic energy conservation solve for velocities
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:
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
Striking a Light Mass Let m 1 >> m 2, when a car 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