KS3 Physics - Relative Motion

In this video we're learning about relative motion, so we'll cover what relative motion is, how to calculate relative motion for objects moving towards each other, and then finally how to calculate relative motion for objects moving in the same direction. Let's start with what relative motion is. Relative motion refers to the movement of one object as observed from a particular frame of reference, such as from another moving object. As an example, imagine you're sitting in a car that's moving at 60 kilometers per hour, and there's another car moving beside you at the same speed. To you it looks like the other car isn't moving at all, even though both cars are moving quite quickly. This is because the speed of the other car relative to you, or in other words its relative motion, is zero. Now if the other car speeds up and moves faster than your car, its relative motion increases and you'll see it moving forwards, whereas if it slows down its relative motion decreases and it will appear to move backwards. Of course in this scenario all the cars are still moving forwards, but relative to you they appear to be stationary, moving forwards, or moving backwards. So next let's take a look at how to calculate relative motion for objects moving towards each other. When two objects are moving towards each other in opposite directions along the same line, we can calculate their relative motion by adding their speeds together. As the objects are moving towards each other, they're getting closer faster than if one were stationary. For example, imagine two trains moving towards each other on the same track. Train A is moving at 10 meters per second and train B is moving at 15 meters per second. To find their relative motion we calculate the speed of train A plus the speed of train B. So that's 10 meters per second plus 15 meters per second giving us 25 meters per second. So the relative speed of train B as seen from train A is 25 meters per second and vice versa, which means that the distance between them decreases by 25 meters every second. To finish up, let's explore how to calculate relative motion for objects moving in the same direction. When two objects are moving in the same direction along the same line, to calculate their relative motion we work out the speed of the faster object minus the speed of the slower object. So for example, let's say you have a train moving at 60 kilometers per hour and a car following it at 80 kilometers per hour. To find the relative motion we take the speed of the car because that's the faster object and minus the speed of the train because that's the slower object. So we do 80 kilometers per hour minus 60 kilometers per hour equaling 20 kilometers per hour. So the car's relative speed to the train means that the car will catch up to the train at a rate of 20 kilometers every hour. If you haven't heard yet, you can find all of our videos on our website cognito.org. You'll also find questions, flashcards, exam-style questions and past papers and we track all of your progress so that you always know what to study next. So sign up for free by clicking here or browse our playlist here on YouTube.