ONLINE Exercise: Collisions Purpose: Investigate conservation of momentum and mechanical energy in a collision. Introduction When objects collide, whether billiard balls, vehicles, shopping carts or football players, the results of their collision can be complicated. Yet even in the most chaotic of collisions, as long as there are no external forces acting on the colliding objects, conservation of linear momentum always holds and provides an excellent tool to study the dynamics of the collision. The conservation of momentum in these closed systems is simply p _i=p _f . In most cases, the collision time is short which means that the effect of external forces will be small in comparison to forces that the objects exert on each other. For a collision between two objects no external forces acting on the objects, the conservation of linear momentum requires m_1 v _1i m_2 v _2i=m_1 v _1f m_2 v _2f . where we have used the definition of momentum, p =mv . A collision is called elastic if both the linear momentum and kinetic energy conserve. A collision is called inelastic if only the linear momentum conserves. Perfectly elastic collisions have another conserved quantity, the kinetic energy. The conservation of kinetic energy