Transcript

What is ‘relative speed’ and why does it matter? Well, imagine you’re a police officer with a speed gun, trying to detect speeding motorists. You park your police car at the roadside, and point the speed gun at the passing traffic. Bingo – you’ve got a reading of their speed. But what if the police car is moving too? Of course, we have to remember that all speeds are relative. And what matters here is what speed the suspect is doing relative to the roadside. In this case, the suspect car isn’t actually going at 150 kilometres per hour. That’s its speed relative to the police car. So, the police car’s speed and direction need to known in order to work out if the suspect car is actually speeding. And it’s a similar scenario if a car is driving away from us. This lines up exactly with our natural intuition. We know that if we hit something coming towards us fast, it’s likely to have much greater impact than if we hit something going in the same direction and at a similar speed. So the maths makes perfect sense.