lesson #13

LESSON #13

collisions

Multiple Choice

2) What letter is typically used to represent momentum in physic equations?

• m

• w

• v

• b

• p

Multiple Choice

Which of the following is a unit of measurement for momentum?

• N

• N m/s

• J

• kg m/s

• J/s

Multiple Choice

Which equation should we use to predict the velocity of the two cars when they collide and stick to each other?

• $$P_{tot}=\left(m_1v_1\right)+\left(m_2v_2\right)$$  

• $$P_{tot\ }=\ \left(m1+m2\right)Vf$$  

Multiple Choice

Which equation should we use to predict the velocity of the two cars when they collide and move away from each other?

• $$P_{tot}=\left(m_1v_1\right)+\left(m_2v_2\right)$$  

• $$P_{tot\ }=\ \left(m1+m2\right)Vf$$  

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.

HOW TO DIFFERENTIATE TYPES OF COLLISIONS?

By the Coefficient of Restitution?

The coefficient of restitution (denoted as " e " is a number that describes how "elastic" a collision is between two bodies

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​What is the Coefficient of Restitution?

The value of e ranges between 0 and 1, where:
- e = 1 : elastic collision
- e = 0 : Perfectly inelastic collision
- 0 < e < 1: inelastic

example.

Two cars with masses m1 = 1000 kg and m2 = 1500 kg collide. Before the collision, m1 was moving at 15 m/s and m2 was at rest. After the collision m1 moves at 5 m/s in the same direction. Calculate the coefficient of restitution and the final velocity of m2.

Differentiating Types of Collisions

Inelastic Collision: Only momentum is conserved; kinetic energy is not conserved. Example: car accident where cars are deformed

Perfectly Inelastic Collision: The extreme case of inelastic collision where the bodies stick together after the collision.

Elastic Collision: Conservation of both momentum and kinetic energy. Example: collision of billiard balls