# Q & A: relativity on a train

Q:
If you released a toy helicopter to hover on a train travelling at say 100mph would the toy slowly loose speed and be overtaken by the train as it is no longer in contact with train? thanks
- Andy (age 30 )
UK
A:
By itself,  the helicopter has no tendency to speed up or slow down. That's called the principle of inertia, and it is closely related to the principle of relativity.

So long as the train is sealed, the toy helicopter will not notice that the earth happens to be moving by quickly beneath the wheels. Remember, that's just as valid a viewpoint as the one that says the earth is standing still and the train moving.

Of course, if the windows are open there may be a breeze going backwards in the train frame. In that case, it will blow the helicopter backward. Notice that this can be described in the train frame as due to a wind blowing on the train or in the earth frame as due to the train moving through the air.

Mike W.

(published on 09/19/2011)