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If we want the slope at t = 0 s, we might take t = 0 s and t + Δt = 2 s. Again, this is numerical differentiation. How can we do it? Well, in most cases, and especially in the case of experimental measurement, we do the same thing that we did for the simple case: we draw a triangle and put change in displacement Δx over change in time Δt. We want to find the slope of this curve, at different values of t, as is shown in the animation. Let's work out his velocity at any particular time t, just from the graph x(t). What if v is not constant? Here's a simple case in which the same bloke is accelerating forwards, so his velocity is increasing. The derivative of x in this case is constant, as we show in the v(t) graph above. That, however, is just for this special case, where v is constant, and so v and are the same. If we made the run 0.3 s, the rise would be 0.3 m, and so on. Note too that, for this case, it doesn't matter how big or where we draw the triangle. Often it is said to be the rise (here Δx) over the run (here Δt). Note the geometrical significance of taking the derivative: looking at the triangle drawn on that graph, the height is 2 m, and the base is 2 s, so the derivative is the slope of the graph. That process is called numerical differentiation, and most differentiation is numerical. Yes, that's all there is to it: we subtracted to obtain a difference, and divided it by another difference. Now, don't get too excited, but what we have just done is called 'differentiation' or 'taking a derivative'. (In a word, δ sounds similar to the 'd' in English, so you can think of it as standing for 'difference'.) With that substitution, and remembering to use average velocity, we write: "change in" occurs so often in physics that we replace it with the Greek letter delta, whose upper and lower case forms are Δ and δ. We'll write x for displacement and t for time. That fraction above looks long and clumsy when written in words. His average speed is written, pronounced 'v bar'. We signify the average of something by putting a bar over it. Let's see how special: What if he travelled at 2 m/s for half a second, stopped for one second, then travelled at 2 m/s for another half second? He would still have travelled two metres in two seconds, so his average speed would be 1 m/s, even if he were never travelling at this speed. Now this is a special case, because in this example he is travelling at constant speed. So what is v? Displacement has increased by 2 m, time has increased by 2 s, so v is When the clock reads t = 2 s, he is at x = 5 m. We call this his initial displacement and write x 0 = 3 m. When the clock strikes zero, he is at x = 3 m. By the way, the magnitude of velocity is called the speed, which we could write as | v|. In these examples, we shall consider only motion in a straight line, so we can specify the direction simply thus: Positive velocity means going to the right, negative velocity means going to the left. So here we could say that his speed is 1 m/s but his velocity is 1 m/s towards the right. ( An aside for physicists: velocity is a vector, meaning that it has direction as well as magnitude. This means that, for each second he travels, his displacement from the starting position increases by 1 m. The strange man in this animation is moving in a straight line at a constant speed of one metre per second. The velocity is the rate of change of displacement. This page supports the Physclips project.ĭifferentiation: How rapidly does something change? What is a logarithm? A brief introductionĭifferential Equations: some simple examples (separate page).Integration: How do the results of a variable rate add up?.Differentiation: How rapidly does something change?.This short introduction is no substitute, however, for a good high school calculus course: we shall take some short cuts of which mathematicians may disapprove. So stick with us: differentiation really is just subtracting and dividing, and integration really is just multiplying and adding. Fortunately, one can do a lot of introductory physics with just a few of the basic techniques. For physics, you'll need at least some of the simplest and most important concepts from calculus. Calculus analyses things that change, and physics is much concerned with changes. The basic ideas are not more difficult than that. The flow is the time derivative of the water in the bucket. Here's a simple example: the bucket at right integrates the flow from the tap over time. Calculus: differentials, integrals and partial derivatives.Ĭalculus – differentiation, integration etc.