This question already has an answer here: A fly in an accelerating car 3 answers

Question

This question already has an answer here:

A fly in an accelerating car 3 answers

Scenario:

You ride in a train, you have this helicopter toy. The train is not yet running when you flew your helicopter on a constant altitude (say 1 meter above the train's floor).

Question: What will happen on the flying helicopter toy when the train accelerates?

Will it stay relative to you and the train?

Will it move backward relative to you and the train and bump to the rear part of the train?

Will it move backward relative to you and the train for few seconds and then stay?

Any other thoughts?

Explanation / Answer

The question Manishearth mentions is certainly closely related.

If the helicopter is flying then it is affected only by the air around it. Experience of riding in trains suggests that when the train starts the air moves with the train - at least I don't recall feeling any air movement when the train starts.

So the answer is either 2 or 3 depending on how fast the helicopter is affected by the air around it. When the train starts the helicopter is initially unaffected so the train passengers see it start to move towards the rear of the train. As the train accelerates the helicopter will feel an increasingly strong wind blowing from the rear to the front of the train, and this wind will start to accelerate the helicopter towards the front of the train. Whether the helicopter accelerates fast enough to avoid being hit by the rear of the compartment depends on how fast the wind accelerates it.

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