0.18 Phy1160: force and motion -- momentum, impulse, and conservation  (Page 6/14)

A rebound collision

A rebound collision involves a change in direction in addition to a change in speed. Because the direction changes, there is a large velocity change even ifthe magnitude of the velocity stays the same.

Elastic collision

Collisions in which objects rebound with the same speed (and thus, the same momentum and kinetic energy) as they had prior to the collision are known as elastic collisions .

Stated differently, an elastic collision is a collision between two bodies in which the total kinetic energy of the two bodies after the collision is equal totheir total kinetic energy before the collision. Elastic collisions occur only if there is no net conversion of kinetic energy into other forms.

Energy conversion

A future module will explain kinetic energy and other forms of energy, such as potential energy in detail. Briefly, kinetic energy is energy possessed by amoving object simply because it is moving. For example, it hurts more to be hit by a fast moving baseball than to be hit by a slow moving baseball, simplybecause the collision with a fast moving baseball imparts more energy into your body. In other words, the kinetic energy possessed by the fast-moving baseballis converted into pain in your body.

Characteristics of an elastic collision

An elastic collision is typically characterized by a large velocity change, a large momentum change, a large impulse, and a large force.

While the case of a Super Ball bouncing on a solid concrete floor isn't a perfect elastic collision, it comes very close. The amount of kinetic energythat is converted into other forms of energy during each bounce is very low, and the ball will continue bouncing for a very long time with the height of eachbounce being almost as high as the height of the previous bounce.

Action and reaction

Two objects collide when they make contact while one or both are moving. As is the case with all interactions involving two or more moving objects, a collisionresults in a force being applied to all of the objects involved in the collision. The behavior of such collisions is governed by Newton's laws ofmotion.

Newton's third law

One paraphrased version of Newton's third law (see (External Link) ) reads:

... in every interaction, there is a pair of forces acting on the two interacting objects. The size of the force on the first object equals the sizeof the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forcesalways come in pairs - equal and opposite action-reaction force pairs.

According to Newton's third law, when two objects are involved in a collision, the two objects experience forces that are equal in magnitude andopposite in direction.

In most cases, the collision will cause one object to gain momentum and the other object to lose momentum. This, in turn, will cause one object to speed upand the other object to slow down.