1. Warm-up Checking HW (Conservation of Momentum Practice)
Did you bring your Egg Drop Supplies? Put your name on it and place it by the front windows
On your warm-up sheet:
What is kinetic energy?

2. Collisions

3. Kinetic Energy
The energy of an object that is due to the object’s motion
Scalar quantity
SI Unit: Joule (J)
𝑲𝑬= 𝟏 𝟐 𝒎 𝒗 𝟐
𝑘𝑖𝑛𝑒𝑡𝑖𝑐 𝑒𝑛𝑒𝑟𝑔𝑦= 1 2 𝑥 𝑚𝑎𝑠𝑠 𝑥 (𝑠𝑝𝑒𝑒𝑑) 2

4. Perfectly Inelastic Collisions
A collision in which two objects stick together after colliding.
Momentum is conserved before and after the collision. Objects become “one” and move at the same velocity ( 𝑣 𝑓 )
𝒎 𝟏 𝒗 𝟏,𝒊 + 𝒎 𝟐 𝒗 𝟐,𝒊 =( 𝒎 𝟏 + 𝒎 𝟐 ) 𝒗 𝒇
Kinetic Energy is NOT conserved.
Examples:
Football player tackles another
Meteorite collides with Earth
Cars crash into each other and are entangled

5. Kinetic Energy--Perfectly inElastic Collisions
Kinetic Energy is NOT conserved!
Some energy converted into heat, sound energy, and internal energy (deforming objects)
∆𝑲𝑬= 𝑲𝑬 𝒇 − 𝑲𝑬 𝒊
Initial: 𝐾𝐸 𝑖 = 𝐾𝐸 1,𝑖 + 𝐾𝐸 2,𝑖 = 1 2 𝑚 1 𝑣 2 1,𝑖 𝑚 2 𝑣 2 2,𝑖
Final: 𝐾𝐸 𝑓 = 𝐾𝐸 1,𝑓 + 𝐾𝐸 2,𝑓 = 1 2 𝑚 1 𝑣 2 1,𝑓 𝑚 2 𝑣 2 2,𝑓

6. Practice
Two clay balls collide head-on in a perfectly inelastic collision. The first ball has a mass of kg and an initial velocity of 4.00 m/s to the right. The second ball has a mass of kg and an initial velocity of 3.00 m/s to the left. what is the velocity of the balls after the collision?

7. Practice
What is the decrease in kinetic energy during the collisions?

8. Elastic Collisions
Objects collide and return to their original shapes (think “elastic” like a rubber band)
Momentum & Kinetic Energy are conserved
𝒑 𝟏𝒊 + 𝒑 𝟐,𝒊 = 𝒑 𝟏,𝒇 + 𝒑 𝟐,𝒇
𝑲𝑬 𝟏,𝒊 + 𝑲𝑬 𝟐,𝒊 = 𝑲𝑬 𝟏,𝒇 + 𝑲𝑬 𝟐,𝒇
Examples:
Billiard balls
Golf gall and club

9. Practice
A kg marble moving to the right at m/s makes an elastic head-on collision with a kg shooter marble moving to the left at m/s. After the collision, the smaller marble moves to the left at m/s. Assume that neither marble rotates before or after the collision and that both marbles are moving on a frictionless surface. What is the velocity of the kg marble after the collision?

10. Collisions in The real world
Perfectly Inelastic Collisions & Elastic Collisions are two extremes.
Most real-world collisions are “Inelastic Collisions”
Objects do not stick together or travel together after a collision
Kinetic Energy is converted to other forms of energy