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?

Collisions

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

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

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 + 1 2 𝑚 2 𝑣 2 2,𝑖 Final: 𝐾𝐸 𝑓 = 𝐾𝐸 1,𝑓 + 𝐾𝐸 2,𝑓 = 1 2 𝑚 1 𝑣 2 1,𝑓 + 1 2 𝑚 2 𝑣 2 2,𝑓

Practice Two clay balls collide head-on in a perfectly inelastic collision. The first ball has a mass of 0.500 kg and an initial velocity of 4.00 m/s to the right. The second ball has a mass of 0.250 kg and an initial velocity of 3.00 m/s to the left. what is the velocity of the balls after the collision?

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

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

Practice A 0.015 kg marble moving to the right at 0.225 m/s makes an elastic head-on collision with a 0.030 kg shooter marble moving to the left at 0.180 m/s. After the collision, the smaller marble moves to the left at 0.315 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 0.030 kg marble after the collision?

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