Each of the four runners ran 400 m, and the clock shows the total time taken. If you look at the speedometer in a car, it doesn’t tell you the car’s average speed ; rather, it tells you its speed at the instant when you look at it. Average speed is calculated over a period of time. If its speed is changing, then the equation gives us its average speed. If the object is moving at a constant speed, this equation will give us its speed during the time taken. m s–1įigure 1.3 The winning team in the men’s 4 × 400 m relay at the World Athletics Championships.
Note that in many calculations it is necessary to work in SI units (m s–1). The 1Ĭhapter 1: Kinematics – describing motion photograph contains enough information to enable us to work out the runners’ average speed. The clock shows the time they took to cover 1600 m – they did it in less than 3 minutes. The photograph (Figure 1.3) shows the US team that set a new world record in the men’s 4 × 400 m relay. Where v is the average speed and x is the distance travelled in time t. We can calculate the average speed of something moving if we know the distance it moves and the time it takes: average speed = A stroboscopic lamp flashes at regular intervals ; the camera is moved to one side at a steady rate to show separate images of the boy.įigure 1.1 Muybridge’s sequence of photographs of a horse trotting. If we knew the time between flashes, we could measure the photograph and calculate the speed of a ball as it moves through the air.įigure 1.2 This boy is juggling three balls. As he does so, a bright lamp flashes several times a second so that the camera records the positions of the balls at equal intervals of time. This is a stroboscopic photograph of a boy juggling three balls. Figure 1.2 shows another way in which movement can be recorded on a photograph. This had been the subject of much argument, and even of a $25 000 bet. He was able to show that, at some times, a trotting horse has all four feet off the ground (Figure 1.1). The Victorian photographer Eadweard Muybridge used several cameras to take sequences of photographs of moving animals and people. Photography has played a big part in helping us to understand movement. It’s important for us to be able to judge movement – think about crossing the road, cycling or driving, or catching a ball. We notice even quite small movements out of the corners of our eyes. Chapter 1 Kinematics – describing motion Describing movement Our eyes are good at detecting movement.
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