標題:
A train and a butterfly
發問:
最佳解答:
When the train accelerates and moves forward, there is an external force acting on the train. However, there is no such external force acting on the butterfly. By Newton's 1st law of motion, it will keep on its state of motion due to its inertia. That is, it will keep stationary at its initial point. But the people on the train is accelerating with the train (The force for acceleration is the frictional force acting on the people by the floor of the train), while the butterfly stays at its initial position. So, from the point of view of the people on the train, the butterfly seems moving backward with an acceleration. But to the other observer in the inertial frame besides the train, the butterfly does not move at all. The observation by the observer in the inertial frame is correct while the observation by the people on the train is invalid since they are not in an inertial frame. 2008-09-11 17:04:34 補充： Yes, but there is a dilation period. 2008-09-11 20:59:50 補充： Maybe I should explain in this way. When the train accelerates, due to inertia, the air will stay where they initial were. As a result, much air seems 'concentrate' at the back end of the train. 2008-09-11 20:59:53 補充： As a result, there is a pressure gradient between the back and the front of the train. Hence, the butterfly is being exerted a force to follow the train to accelerate due to the pressure difference.
其他解答:
A train and a butterfly
發問:
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Inside a resting train, there is a butterfly flying. The butterfly does not move at all. It is at a point in the air. Then the train accelerates and moves. Will the butterfly suddently move backwards if the train is moving forward? Thanks 更新: So the the butterfly stays at the initial position? Does the air inside the train does anything to move the butterfly? 更新 2: When the train accelerates more dense air is built up at the back end of the train. This results the butterfly being pushed by the air density even the is not very dense. Am I right?最佳解答:
When the train accelerates and moves forward, there is an external force acting on the train. However, there is no such external force acting on the butterfly. By Newton's 1st law of motion, it will keep on its state of motion due to its inertia. That is, it will keep stationary at its initial point. But the people on the train is accelerating with the train (The force for acceleration is the frictional force acting on the people by the floor of the train), while the butterfly stays at its initial position. So, from the point of view of the people on the train, the butterfly seems moving backward with an acceleration. But to the other observer in the inertial frame besides the train, the butterfly does not move at all. The observation by the observer in the inertial frame is correct while the observation by the people on the train is invalid since they are not in an inertial frame. 2008-09-11 17:04:34 補充： Yes, but there is a dilation period. 2008-09-11 20:59:50 補充： Maybe I should explain in this way. When the train accelerates, due to inertia, the air will stay where they initial were. As a result, much air seems 'concentrate' at the back end of the train. 2008-09-11 20:59:53 補充： As a result, there is a pressure gradient between the back and the front of the train. Hence, the butterfly is being exerted a force to follow the train to accelerate due to the pressure difference.
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