 So we've learned about forces, which are one way to figure out how things behave in classical system And if you know that all the forces you can work out what's going to happen in the future that can be quite complicated though Simple ways of working things out involved looking at conserved quantities And the first one we saw was momentum now momentum is always conserved. It only has one form The only disadvantage is it's a vector So it has three components it's got a magnitude and a direction And so when you write down a conservation of momentum what you're really writing down is three conservation laws The conservation of momentum in the x-direction, the conservation in the y-direction and the conservation in the z-direction all in one hit A simpler quantity that only has a scalar quantity is something like a charge and charge is always conserved But problems that require us to use the conservation of charge don't turn up that often except in more exotic situations So while being completely true and very important, it's not very useful There's mass, mass is just a scalar, but it's not always conserved. It's mostly conserved That said at non relativistic speeds and so forth when you're not worrying about nuclear reactions Mass is effectively conserved So you can use things like conservation of pens or conservation of storm water if a lot of water goes into a drain You can figure out how fast the water has to be coming out the other end You can work out traffic flow and so forth using conservation of mass. That's a good rule of thumb And we've also learned about conservation of energy. Now energy is important in lots of everyday situations It's a scaler. So that's very handy and it's always conserved, which is great Any problem is it comes in lots of forms and so to use it You have to learn all the different forms of energy and track how energy travels from one to another And in fact this tracking of how energy and learning how to use energy is one of the main things that enabled us to change our Technology over the last few hundred years from all the millions of years of fairly static technology beforehand