Consider that i'm travelling in a train which runs at a speed of 120kmph,i see a fly flying inside the train..

Consider that i'm travelling in a train which runs at a speed of 120kmph,i see a fly flying inside the train.. Will the fly be travelling at 120kmph ? Pls Explain !

relative to you, it will be traveling at a normal speed.

relative to an earth reference, it is traveling at 120 km/h, plus or minus it's flight speed.

Yes, the fly is travelling at 120kmph. It is just the question of relativity. If you are outside on the tracks, and teh train passes you at 120kmph, then the fly is also travelling at 120kmph relative to u. But if u are in the train, you and and fly at travelling at the same speed, so the fly is travelling at 0 kmph relative to you.

THIS IS A SILLY QUESTION

If the train was 120 km long in an hour the fly would only be 20km down the train.
and also the fly could be going the opposite way to the train.

relative to an observer standing on the side of the tracks, yes.

relative to you. no.

the observer sees the fly's velocity as the difference between his velocity and the (velocity of train + velocity of fly). from your frame of reference, which is already traveling the velocity of the train, you see only the difference between your velocity and that of the fly.

since your on the subject, here's another thought.

to an observer standing still on the side of the tracks, the fly has a forward velocity. if the fly decides to freefall to the floor of the train, he would do so in a projectile motion accelerating towards the floor and his path would be curved. exactly like a typical projectile.

to an observer sitting on the train, the fly appears to fall in a straight line down.

how can his path be both curved and straight at the same time?