Collisions and Impulses

2. Impulse and Collisions

If you have more time to brake, you can do it more gently (slowly pressing the brake instead of slamming on it really quickly)...

... which is a lot more comfortable for the passengers in your car (and yourself) because you experience a weaker force

Let's look at a slow mo video of a soccer ball hitting somebody's face

It's the same concept as the boxer and the fist. Normally, when a soccer ball comes flying at your face, you would try to move WITH the motion of the ball, so that it's in contact longer (greater Δt) but less forcefully (smaller F)

Multiple Choice

True or false: an airbag doesn't protect you because it's soft (it's really not soft at all!!!!), it protects you because it extends impact time and therefore reduces the impact force

• True

• False

You can find these plastic covering on most medians

They're designed to crumble when a car hits them, instead of hitting the solid metal

Multiple Choice

When one object exerts an impulse on another object, how do you minimize the damage done?

• Maximize contact time Δt to minimize the force F

• Minimize contact time Δt to minimize the force F

• Minimize contact time Δt to maximize the force F

• Maximize contact time Δt to maximize the force F

Multiple Choice

According to Newton's Third Law of Motion,

• an object in motion will stay in motion until a force changes that

• F=ma

• every action force has a reaction force which is equal in magnitude and opposite in direction

Multiple Choice

Which is an example of Newton's Third Law?

• A baseball bat hitting a ball and the ball flying away

• A baseball bat hitting a baseball and the baseball hitting the bat in return

Collisions in Physics

There are 3 different types of collisions

Write them down as follows

Multiple Choice

True or False: In a perfectly elastic collision, the two colliding objects will separate again afterwards without any damage or change of shape

• True

• False

Ok, so I put the (Perfectly) in parenthesis because there really is no such thing as a perfectly elastic collision in the real world. Why?

• Collisions involve transfer of energy from one object to the other

• In a "perfect" situation, zero energy would be wasted to sound or heat, but in reality, a collision will always make a sound and leave the colliding surfaces a bit warm

• So most collisions in the real world are "just elastic": the objects aren't damaged or change shape

Multiple Choice

When two cars crash head on, it's only a perfectly inelastic collision if they get tangled up and move as one giant lump of metal afterwards

• True

• False

Multiple Choice

Two people on ice skates are standing still, touching hands and then push off of each other, resulting in both of them sliding backwards. According to the definition I gave you, is this considered an "explosion" in physics?

• Yes because they start as one "object" at rest and then separate into two moving objects

• No... just... no