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## Relative velocity in 1D

# Intro to relative velocity

## Video transcript

in this video we're gonna talk about something called relative velocity this just tells us that the velocity that you measure for any object really depends upon the point of view let's try to understand this with an example let's say that you are standing over here on the ground it's a beautiful day you're enjoying the scenery and suddenly you see a truck zooming past you extremely fast something like this and let's say you calculate its velocity and you find out that its velocity is let's see let's say the velocity of the truck turns out to be a hundred kilometers per hour now what you want to do is you get excited by this by this trucks motion and so you decide to get into the car and start driving yourself so what you do now is you get into the car so let's put you inside the car here you are alright and suppose you start driving let's say with the velocity of let's say your car doesn't go quite as much as this guy but let's say it goes at 95 kilometers per hour now here's the question now that you are inside this car suppose you again look at this truck what will you see the velocity of the truck to be do you still seem to be hundred or do you see it to be faster or slower think about this I want you to pause this video for a while and think what would you see in this situation the velocity of truck to be and this could be a familiar situation for you you may have experienced this let's look at things from the cars point of view let's see this is what would look like once you are inside the car all you could see is the seat of the car and the window of the car and you could see outside that truck the first thing I want you to note is that once you are inside this car you don't see that car moving anymore I mean it might shake a little bit but that car is not moving as seen by you the seeds the window if there are any passengers over there they just stay there and now because of this we can say that the speed of the car or the velocity of the car as seen from your point of view now once you're inside the car that velocity is zero in other words you are at rest let me write that down you are at rest your car is at rest as seen by you but instead when you look outside what do you see well now you see the ground whizzing backwards you know you something like this something like that you see the ground whizzing backwards and let's think about this see as seen from the ground that the car is going forward 95 kilometers every hour so I hope this makes sense that as seen from the car you would now see the ground going backward at 95 km/h the ground goes backward at 95 km/h and we can now sort of understand what's going on over here you see the truck is traveling on the ground 100 kilometer every hour forward but the ground itself is going backwards 95 kilometers every bar this is like this is like a conveyor belt that is going backwards an extremely high speed and some kid is trying to run on that I hope you see that because of this that the kid will find it extremely difficult to move forward and the same thing is going to happen for the truck the truck keeps struggling to go forward because the ground is constantly moving back so the question now is if I wait for one hour how much has the truck gone forward well we can calculate that so if you wait for one hour you will see that the truck has gone forward forward by a hundred kilometers on the ground so let's write that down let's use this point to measure the position of the truck so notice that the truck would have gone forward a hundred kilometers in one hour so this is 100 kilometers per hour but in that one hour you would also see because the ground is going back you would see both the truck and the ground travel back 95 kilometers so it comes somewhere maybe somewhere over here travels all the way back till here maybe you would see that the truck and the ground they all go together backward till here till here let's say that's 95 kilometers per hour and now I hope you can see that in one hour effectively the truck has only gone forward let's see how much is that I will mark it over here how much has the truck gone forward in one hour 100 minus 95 only five kilometers per hour and therefore as seen from the car the truck only goes forward five kilometers every hour so we could now say that the velocity of the truck when you look at it from the car is five kilometers per hour forward so this is summarise what happened when we look from the ground why am i looking by standing on the ground and over here the the truck is going for 100 km/h but from the car when you look at it it's only 5 km/h so do you see that the velocity of the truck that you measure really depends upon where you measure it from if you're measuring it from ground you get one value you measure it from the car you get another value that's the whole idea behind relative velocity velocity is a relative term all right and one last detail I want to tell you a technical term that you know people usually use when you're talking about relative velocity is they use the word reference frame so instead of saying that the velocity of the truck as measured by someone standing on the ground is hundred kilometres per hour they would just say they would just say that the velocity of the truck from the ground reference frame that's the technical term we used over there so we could say the velocity of the truck from the ground reference frame is hundred kilometers per hour and similarly over here instead of saying that you know the velocity of the truck from the car is assisting from the car is PI kilometers per hour they would say the velocity of the truck from the car reference frame so we will call this as the car reference frame I want to show you this exact situation take a look at this video I shot this video from inside a car and first of all I don't see my car moving at all but instead notice the whole ground is traveling backwards very fast and I'll take a look at this truck the truck is going pretty fast with respect to ground but notice that with respect to me it's struggling to go forward its velocity as seen by me he's extremely slow can you see that I hope this can win you that velocity is truly a relative concept