 Speed and acceleration are physics concepts that we encounter every day, and an understanding of these concepts is crucial to most areas of physics. We think of speed as something we have when moving, and this forms the basis of speed's definition. Speed can be defined as the rate of change of distance. What exactly does that mean? Well, it means that an object's speed is a measure of how long it takes in terms of time for it to travel a given distance. From this, we get the following equation. Speed equals distance divided by time. The standard units of distance is meters, and time is seconds. Because of this, speed is measured in distance per time. For example, meters per second, or kilometers per hour. Let's look at an example. A car travelled 100 km in two hours. What was its speed? So 100 divided by 2 equals 50 km per hour. Now you try. A sprinter runs 100 meters in 12.5 seconds. Pause the video while you work out her speed. Did you get 8 meters per second? And another one. A jet can travel at 350 meters per second. How far will it travel at this speed in 30 seconds? Pause and have a think. Well, it travels 350 meters in one second. So in 30 seconds, it will travel 30 times 350, which equals 10,500 meters. So how far will it travel in three minutes? Pause and have a think. Well, three minutes is 3 times 60, or 180 seconds. So the distance it will travel is 180 times 350, which equals 63,000 meters, or 63 kilometers. In the same way that speed is the rate of change of distance, acceleration is the rate of change of speed. This means that the acceleration of an object is a measure of how much its speed changes over a given period of time. To calculate acceleration, we use the following equation. Acceleration is equal to change in velocity divided by time. Let's try an example. The velocity of a bus increased from 0 to 18 meters per second in six seconds. Calculate its acceleration. So acceleration equals final velocity minus initial velocity over time. Another example. A hungry cheetah saw a gazelle and decided to chase it. The cheetah was walking at 5 meters per second and accelerated to 20 meters per second in five seconds. What was its acceleration? Pause the video or you work it out. The sign in front of an object's acceleration indicates whether the object's speed is increasing or decreasing. If an airplane's acceleration is negative 40 meters per second squared, its speed is decreasing. The gradient of a line on a speed time graph represents acceleration. If the gradient is positive or goes up, the object has a positive acceleration. If the gradient goes down, the object has a negative acceleration or deceleration. What's the acceleration of this object between points A and B? The change in speed is 25 minus 5, which equals 20. The change in time is 6 minus 2, which equals 4. So the acceleration is 20 meters per second over 4 seconds, which equals 5 meters per second per second. Is the acceleration large, small or zero in these time graphs? And is it positive or negative? Pause the video while you think. So that's how speed and acceleration are related and can be calculated. If you liked the video, give it a thumbs up and don't forget to subscribe, comment below if you have any questions. Why not check out our Fuse school app as well? Until next time.