Collisions and ExplosionsMomentum and Energy

Let’s think about collisions: How can 2 objects collide, and what can

happen when they do? Head-on, Rear-end, both moving, only one

moving… They could bounce off each other They could stick together Anything else?

Let’s start with 1 object… Colliding with a wall

What’s happening during that collision? How can we quantify values, such as the

impact force, experienced during collision?

Impulse-Momentum!

During a collision between 2 things:

The time the two objects are in contact with each other is exactly the same

The Force each object experiences is exactly the same (Newton’s 3rd law of motion!)

What does that tell us about the objects’ changes in momentum?

They’re the same magnitude (but in opposite directions)!

Let’s go one step further…

What does this equation tell us, conceptually?

Conservation of Linear MomentumAs long as there is no outside force acting on the objects, within a system of masses the total vector sum of their momenta must remain constant.

In other words, momentum must be conserved

Total momentum BEFORE a collision or explosion (an event) will be the same as the total momentum AFTER the collision or explosion

Impulse-Momentum will give us a picture of what happens DURING the collision

Example: A rail truck of mass 4.50 x 103 kg is

moving at a speed of 1.80 m·s-1 when it collides with a stationary truck of mass 1.50 x 103 kg. The two trucks couple together. What is the velocity of the trucks immediately after the collision?

Example 2 Stone A of mass 0.50 kg is sliding at 3.8 m·s-1

across the surface of a frozen pond when it collides with a stationary stone B of mass 3.00 kg. After the collision, stone B moves off at a speed of 0.65 m·s-1 in the same original direction as stone A’s initial velocity. What is the final velocity of stone A?

Elastic Collisions Objects bounce off each other with no

loss of energy (i.e. no damage or deformation has been done to either object)

Total momentum remains constant Total kinetic energy remains constant

Examples: atoms, magnetic bumpers

Inelastic Collisions Momentum is conserved Kinetic energy is NOT conserved Objects tend to use some of the original

energy for compression, sound, breaking, heating up, etc.

Examples: bug hitting a windshield; baseball bat against baseball, etc.

Perfectly Inelastic Collisions Momentum is conserved Objects stick together upon impact Kinetic energy is not conserved