Elastic and Inelastic Collisions

Elastic and Inelastic Collisions

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In the elastic collisions from our lab, the 2 carts bounced off each other and went in the other direction

Multiple Select

Was momentum conserved with the elastic collisions?

• Yes!

• NO

In the inelastic collisions from our lab, the 2 carts did NOT bounce off each other, they stuck together and went in the same direction

Multiple Select

Was momentum conserved with the inelastic collisions?

• Yes!

• NO

Open Ended

If momentum was conserved in both collisions, what do you think caused the carts in the inelastic collision to stick together, and not bounce?

With elastic collisions, kinetic energy is conserved. With inelastic, kinetic energy is not conserved (some is lost)

Elastic

Like 2 pool (billiard) balls colliding, they bounce off each other.

Not many collisions are truly elastic

Think about this...

If we kick a soccer ball perfectly straight on, it will go far.

This would be perfectly (or as close to perfect as possible) ELASTIC

If we kind of miss a little and not kick the ball perfectly, it won't go as far

This would be INELASTIC, which is what most collisions are like.

Poll

Can you kick a soccer ball perfectly every time, or do you think most times some of that energy is lost?

• Perfect every time!

• I think most times some energy is lost

Inelastic

Like 2 cars crashing into eachother, they do NOT bounce (I hope) and a lot of kinetic energy is lost to sound and heat.

A lot of external (outside) forces can make the kinetic energy in a collision not conserved, making it inelastic.

• Friction

• Air resistance

• Gravity

• The type of object colliding

Open Ended

Car crashes are often approximated to be perfectly inelastic. What do you think this means?

Perfectly Inelastic collisions

Maximum amount of kinetic energy is lost, causing the 2 objects that collided to stick together. This is what happened in your simulation, and when to cars collide head-on.

Multiple Select

Bouncing a tennis ball off the ground straight down and it bounces back straight up.

• Elastic

• Inelastic

• Perfectly inelastic

Multiple Select

2 cars crash, the first car hits the second car on the side, causing the second car to spin off the road.

• Elastic

• Inelastic

• Perfectly inelastic

Multiple Select

2 cars crash head-on, causing them to smash together.

• Elastic

• Inelastic

• Perfectly inelastic

Why are collisions inelastic at all? Because of the things colliding.

A rubber band bounces back to its original position when stretched (elastic). A car does NOT bounce back.

Back to perfectly inelastic...

There's an equation we can use!

Open Ended

 $m_1v_{1i}\ \ +\ m_2v_{2i}\ =\ \left(m_1+m_2\right)\cdot v_f$  

Remember the car going 15 m/s is in the opposite direction, so V2i = -15