Hi, in this session we will understand the concept of Relative Speed. What really is a Relative Speed,

let us understand it by using this example. Let’s say this blue colour train is waiting, it is stationery at

a particular station. There is a red colour train that is trying to cross it, something like this. And let’s

say it takes t seconds to cross the stationery train. Now what would have happened had this train

not been stationery but moving instead. Right, you would realise that there would be two different

situations, one, when the blue train is moving in the same direction as the red train, in this case you

would have realised that the red train would take more than t seconds to cross, right. On the other

hand, had the blue train been travelling in an opposite direction to that of the red train, then the red

train would cross the blue train like a whisker, right, it would take less than t seconds to cross, right.

Now both these situations are practical situations, you would have come across these situations

quite often, right. Now the big question is, why does this happen? It happens because of this

concept of Relative Speed. So when does relative speed come into play. When two or more objects

move together, right, so the Relative Speed is, when two or more objects move together, Speed of

one object in relation to that of the other, right, speed of one in relation of that to the other. So,

let’s take a practical situation, when will two objects move in the same direction, let’s say, think of a

situation in everyday life. Let’s say, police chasing a thief, right, they are in the same direction. Let’s

put some values to understand this better. Let’s say the separation between them is hundred

meters, the police is travelling at 25 meters per second and the thief is running at 5 meters per

second, right. After one second what would happen, the police would have moved further by 25

meters, whereas the thief would have moved further by 5 meters, right. So right now the separation

between them is 80 meters, so do you realise that in one second, they have effectively bridged a gap

of 20 meters, right, they were 100 apart earlier, now it is only 80. They have bridged a gap of 20

meters in one second, and that’s what is Relative Speed. Right, so in the same direction, the Relative

Speed is given by the difference of the two speeds, 25 minus 5 is 20. So, in the same direction the

Relative Speed will be given by the difference of the two speeds. So if the Speed are S1 and S2, the

Relative Speed will be S1 minus S2. How much time would the police take to catch the thief?