Collisions and Explosions Momentum 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…

During a collision between 2 things:

Let’s go one step further…

Conservation of Linear Momentum As 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 10 3 kg is moving at a speed of 1.80 m·s -1 when it collides with a stationary truck of mass 1.50 x 10 3 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