if a train is travelling with a speed equal to light and a girl is running inside it in the way the train is running.an observer see the girl from outside running.what is the speed of the girl AND WHY??
In order to determine the speed of the girl, we need to consider the concept of relative velocities. The observer who is watching the girl is stationary, while the train is moving with a speed equal to light. Now, since the girl is running inside the train in the same direction as the train's movement, her speed relative to the observer will depend on both her speed relative to the moving train and the speed of the train itself.
Let's assume the speed of the girl relative to the train is "v_g" and the speed of the train is "v_t" (which is equal to the speed of light in this case).
To calculate the speed of the girl as observed by the external observer, we need to add the velocities vectorially. Since the girl is running in the same direction as the train, we can add the velocities:
Speed of the girl (relative to the observer) = Speed of the girl (relative to the train) + Speed of the train
Therefore, the speed of the girl relative to the observer would be: Speed of the girl (relative to the observer) = v_g + v_t
However, since the train is already traveling at the speed of light (which is the maximum speed limit in the universe according to the theory of relativity), any additional velocity (v_g) will still result in the total velocity being equal to the speed of light. Hence, the speed of the girl, as observed by the external observer, will also be equal to the speed of light.
In summary, the speed of the girl observed by the external observer will be equal to the speed of light, because the train is already moving at that speed, and any additional velocity (from the girl running) will not change the overall speed.
The speed of the girl, as observed by an external observer, would be the sum of her speed relative to the train and the speed of the train relative to the observer.
Since the train is traveling at the speed of light, it would be impossible for the girl to be running inside it at any speed. According to Einstein's theory of special relativity, nothing with mass can travel at or exceed the speed of light. Therefore, the concept of a train traveling at the speed of light is not physically possible.