Sana sits by the window of a train standing at a station…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Sana sits by the window of a train standing at a station. The train on the next track starts to move, and for a few seconds Sana feels her own train is moving backward.
- Which train is really moving, compared to the ground?
- Why did Sana feel her train was moving?
- Name a reference point for which Sana is at rest, and one for which she moves, when her train runs at .
Need a reminder? The key ideaShow
Rest and motion are always measured against a reference point. The same object can be at rest compared to one thing and moving compared to another.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) The other train. Compared to the platform, Sana's train is still.
Step 2. (b) She used the other train as her reference point without noticing. Compared to it, her train seemed to go the other way.
Step 3. (c) At rest compared to her seat; moving at compared to the platform or the trees.
Answer: The other train moves; she judged against the wrong reference point.
Check. Looking at the platform settles the question at once.
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