 So far we've talked about the motion of objects by analysing the forces acting on them. Now sometimes we won't be able to measure or even know about all the forces acting on an object, but we'd still like to say something quantitative and predictive, and this is where the much more general concept of energy can be useful. No doubt you've already heard about different kinds of energy, chemical energy, electrical energy or nuclear energy for example. But what is energy really? Well it's actually just a kind of accounting system that lets us keep track of interactions between objects and physical systems. A little bit like money lets us keep track of the exchange of goods and services in a social and economic system. But a really big difference between energy and money is that energy is conserved. The total amount of energy in the universe stays the same and it can only be transferred from one attribute of a system to another attribute or from one system to another. So for example an accelerating car transfers chemical energy into heat by burning petrol and then some of this heat energy is transferred into making the car go faster or increasing its kinetic energy. If the car drives up a hill then some of the chemical energy is used to move the car and everything in it further away from the centre of the earth. This means the chemical energy of the petrol has been converted into what's called gravitational potential energy. If the driver now stops and gets out of the car and drops a ball it will fall and change this potential energy into kinetic energy. Now this idea of energy and keeping track of its different forms and how it changes from one form to another is already a very powerful conceptual framework. But as physicists we need to make it quantitative. We need numbers and equations that tell us how to determine those numbers from things we can measure.