Elastic and Inelastic Collisions

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# Elastic and Inelastic Collisions - PowerPoint PPT Presentation

Elastic and Inelastic Collisions. Chapter 6 Section 3. Collisions. There are many different collisions in which two objects collide. Sports Vehicles Arrow and target. Kinetic Energy and Collisions.

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### Elastic and Inelastic Collisions

Chapter 6 Section 3

Collisions
• There are many different collisions in which two objects collide.
• Sports
• Vehicles
• Arrow and target
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.
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
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.
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.
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.
Kinetic Energy Equations

KElost = KEi – KEf

Kinetic Energy Lost = Initial Kinetic Energy – Final Kinetic Energy

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?
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
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.
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.
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.
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.