Major purpose of this lecture is to present on Physics of Body Crashing. If a body of mass m is in constant motion no acceleration or deceleration occurs! Acceleration a can be caused by leg muscle force F! Deceleration can be caused by friction, muscle force or external forces (by running into a wall for example). There are two general types of collisions in physics: elastic and inelastic. An inelastic collisions occurs when two objects collide and do not bounce away from each other. Momentum is conserved, because the total momentum of both objects before and after the collision is the same.