 In this video, I'm going to explain something that may seem fairly obvious and intuitive, but is a very important concept in physics, and can be useful in simplifying some of the maths. This concept is the concept of ground. If we are investigating or using the amount that a quantity is changing, then there are many cases in which we may not care what the absolute value of the quantities are. In these cases, it can be easier to select a reference point from which to measure quantities. A perfect example of this is temperature units. Typically, when we measure temperature, we use Celsius. This has convenient reference points. At zero, water turns into solid or ice. And at 100 degrees, water boils and turns into steam. So we've set up these two reference points. However, if we want to use absolute values, we'll use temperature units of Kelvin. And there are many problems where we do need to use Kelvin. But there are also many problems where we're only interested in the difference. And in these cases, we typically use Celsius units. Temperature units are a good example of both absolute and relative values. But if we look at distance, there is no absolute reference point. When we measure distances, we always have to specify where we are measuring from, as well as where we are measuring to. So how does this link into our concept of ground? Consider standing on a table, holding a ball three metres above the ground, and dropping this ball to your friend standing in a nearby hole who catches the ball one metre below the ground. Let's find what the change in potential energy of the ball is. While if we do this, using our typical ground level being at the ground, we find that our potential energy is equal to mg times the change in height, which is equal to 3 times mg minus negative 1 mg, which is equal to 4 mg. However, we can also choose to place our reference point where our ground at minus 1 metres. We know that the ball is initially at a height of 4 metres, and if we plug these numbers into our potential energy equation, we find that 4 mg is just 4 mg, and we have our result. So this seems pretty obvious, and it doesn't seem like we've actually simplified the maths, but this is just an example of how we can set our ground at different points to make our lives easier. So what are some ground rules for choosing reference points? Well firstly, reference points are system specific, so comparisons between values need to use the same reference point. If I'm using the ground that's at my feet, and somebody else is using a ground in a hole one metre below the ground, then we need to be aware of this if we're trying to compare values from the different systems. Another helpful hint for using reference points is that an obvious candidate for selecting a reference point in your system is to choose the lowest or highest point in your system. This can really simplify the maths. So for example, if earlier we'd chosen our ground to be 60 metres above the ground, then the person on the table would have been minus 57 metres, and the person on the ground would have been minus 61 metres. That just leads to some fairly ugly numbers. Whereas setting our zero in the hole gave us very nice simple numbers. So why have I called this concept ground? Well the physical ground is a natural reference point in equations, particularly in potential energy, because it's usually the surface that things drop to. You will also come across the term ground when analysing electrical circuits. This ground can also refer to a physical earth or ground connection, and or to a selected voltage reference point in your circuit. As an aside, if you continue on to study quantum mechanics in any detail, you'll come across the ground state, and this term does define the lowest energy state of a system.