Lesson 7: Collisions, Banging Things Together

7. Lesson 7: Collisions, Banging Things Together#

Collisions and Conservation of Momentum

How do we understand the constituents and rules of the universe at its most fundamental level? We have to take it apart. But not the normal way, slicing with a sharp knife won’t do. Rather, we collide elementary particles together and see what comes out. The best analogy of this is learning how old clocks work by smashing them together and collecting the gears and springs and where they landed. We’d hypothesize what the insides of clocks would be like and mathematically model how those insides would be jostled about and ejected when we simulate how they collide. When we get a match, we’d declare that the insides of actual clocks are like the insides of those clocks that gave the best agreement of the debris.

We do the same thing at “particle accelerators” at large and small laboratories around the world. Our detectors and electronics identify potentially interesting collisions and our computer codes sift through the post-collision debris and try to identify tell-tale signs of particles that we know, and maybe evidence for particles that are new.

One of the most important recipes we use to evaluate what comes out of these collisions is “momentum conservation.” We’ll meet this most basic rule of mechanics in this lesson and apply it throughout our time together in QS&BB.

Recall that Descartes imagined that the “in the beginning” there was a given amount of motion in the universe and that the total of it still adds up to that original amount. It’s been passed on through everyday interactions of planets and people and…stuff. He referred to it being “preserved,” but today we refer to it being “conserved.” In some ways, he was almost right and the idea eventually became concrete in the hands of Isaac Newton in Britain and Christiaan Huygens in the Netherlands. Let’s meet that Dutch lens-maker and Foreign Member of the British Royal Society.

  Goals of this lesson:

Understanding, Appreciation, and Familiarity

I’d like you to Understand:

  • The meaning of Momentum Conservation

  • How to use the momentum conservation equation in one dimension

  • How to draw the Feynman Diagram for collisions of two objects

I’d like you to Appreciate:

  • How momentum conservation works graphically in two dimensional collisions

I’d like you to become Familiar With:

  • The history of understanding collisions

  • Huygens’ life