1. Elastic and Inelastic Collisions
Chapter 6 Section 3

2. Collisions
There are many different collisions in which two objects collide.
Sports
Vehicles
Arrow and target

3. Kinetic Energy and Collisions
Momentum is always conserved in a collision, but the total kinetic energy is generally not conserved.
Some of the energy is converted to thermal energy (heat) and internal elastic potential energy when the objects deform.

4. Perfectly Inelastic Collisions
Perfectly Inelastic collisions – A collision in which two objects stick together and move with a common velocity after colliding.
Examples:
Arrow hitting a target
Bullet lodging into a wood block
Meteorite colliding with Earth and becomes buried

5. Perfectly Inelastic Collisions
m1v1i + m2v2i = (m1+m2)vf
Since the objects stick together after the collision, the masses must be added together for the final velocity.

6. Distinctions Between Collisions
Elastic Collision – Objects maintain their original shape and are not deformed after colliding.
Inelastic Collision – Objects are deformed during the collision and lose kinetic energy.
Perfectly Inelastic Collision – Objects join together after a collision to form one mass.

7. Kinetic Energy Lost
Energy is lost during an inelastic collision and not a elastic collision.
In most cases energy is lost during a perfectly inelastic collision, but not always.
How much deformation and how the objects stick together play a factor.

8. Kinetic Energy Equations
KElost = KEi – Kef
Kinetic Energy Lost = Initial Kinetic Energy – Final Kinetic Energy

9. Example Problem
A clay ball with a mass of 0.35 kg hits another 0.35 kg ball at rest, and the two stick together. The first ball has an initial speed of 4.2m/s
What is the final speed of the balls?
Calculate the decrease in kinetic energy that occurs during the collision.
What percentage of the kinetic energy is converted to other forms of energy?

10. Example Problem Answers
2.1m/s
1.6J
52%

11. Elastic Collisions
Elastic Collisions – A collision in which the total momentum and the total kinetic energy remains constant.
The objects remain separate after the collision.
Examples:
Kicking a soccer ball with your foot
Hitting a baseball with a bat
Billiards

12. Everyday Collisions
Most collisions are neither elastic or perfectly inelastic in everyday activities.
In most collisions, kinetic energy is lost.
This places them into the category of inelastic collisions.

13. Kinetic Energy and Elastic Collisions
Kinetic energy is conserved in elastic collisions.
The total momentum and the total kinetic energy remain constant through out the collision.

14. Momentum and Kinetic Energy Equations
m1v1i + m2v2i = m1v1f + m2v2f
Momentum equation can be used for all collisions.
½m1v1i²+ ½m2v2i²= ½m1v1f²+ ½m2v2f²
Kinetic Energy equation can only be used for elastic collisions.

15. Making Sure Collisions Are Elastic
To check and see if a collision is an elastic collision:
Solve the problem using the conservation of momentum equation.
Plug the velocities into the conservation of kinetic velocity equation and see if the total initial velocity and the total final velocity are equal.
If they are, then it is a true elastic collision.