Collisions: Elastic & Inelastic Unit 5.3

Collisions You notice collisions day to day without giving much thought to them.

Collisions There are those in which 2 objects collide and stick together such as an arrow hitting a target. The momentum after would equal their combined momentum before the collision.

Collisions In other collisions, the objects collide and bounce so that they move away with two different velocities such as a tennis ball and a racket. Regardless of the type of collision, the total momentum remains constant.

Collisions However, the total kinetic energy is generally not conserved in a collision because some kinetic energy is converted to internal energy when objects deform.

Collisions In this section, we will examine different types of collisions and determine whether kinetic energy is conserved in each type.

Collisions We will first focus on two extreme types of collisions: elastic & perfectly inelastic.

Perfectly Inelastic Collision When 2 objects collide and move together as one mass, the collision is called perfectly inelastic, such as an arrow and target.

Perfectly Inelastic Collision These types of collisions are easy to analyze in terms of momentum because the objects become essentially one object after collision.

Perfectly Inelastic Collisions The final mass is equal to the combined masses of the two objects and they move with the same velocity after colliding.

Perfectly Inelastic Collision m1v1i + m2v2i = (m1 + m2)vf When using this equation it is important to pay attention to signs that indicate direction.

Perfectly Inelastic Collision In an inelastic collision, the total kinetic energy does not remain constant when objects collide and stick together.

Perfectly Inelastic Collisions Some kinetic energy is converted to sound energy and internal energy as the objects deform during collision.

Collisions This phenomenon helps makes sense of the special use of the words elastic and inelastic in physics.

Collisions Think of elastic as something that returns to or keeps its shape. Think of an inelastic collision producing objects that deform and lose kinetic energy.

Collisions Therefore in an elastic collision, two objects collide and return to their original shapes with no change in total kinetic energy. After the collision, the two objects move separately.

Collisions m1v1i + m2v2i = m1v1f + m2v2f In an elastic collision, both the total momentum and total kinetic energy remain constant throughout the collision. m1v1i + m2v2i = m1v1f + m2v2f

Collisions Remember that [v] is positive if an object moves to the right and negative if it moves to the left.

Collisions In the everyday world, most collisions are not perfectly inelastic or elastic. Any collision that produces sound is not perfectly elastic, the sound representing a decrease in kinetic energy.

Inelastic Collisions Elastic and perfectly inelastic collisions are limiting cases, most collisions actually fall into a category between these two extremes: inelastic collisions.

Inelastic Collision In an inelastic collision, the colliding objects bounce and move separately after the collision but the total kinetic energy decreases in the collision.

Inelastic Collision For our purposes, we will consider all collisions in which the objects do not stick together to be elastic collisions.

Inelastic Collisions This means we will assume that the total momentum and the total kinetic energy remain constant in all collisions that are not perfectly elastic.

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