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Relative Velocity
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This was inspired by a visit to a large shopping mall with criss-crossing escalators. The idea is to discuss the concept of relative velocity by imagining yourself riding on one of the escalators and watching someone on the other.
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The green and blue dots are riders on the respective up and down escalators relative to an observer on the ground. The red dot models green’s motion relative to blue. You have to imagine very smooth and very long escalators. Green is starting at the bottom of the up escalator and blue is at the top of the next floor, going down.
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In the case set up both escalators are at the same angle to the horizontal and travelling at the same speed.
- Run the model and explain why the green and blue dots meet in the middle between the floors. At this instant the red dot is at the blue dot’s initial position, why?
- The up escalator angle is, say, only 25 degrees but its speed is still 4 m/s. If the down escalator is still at 30 degrees what must its speed be now if riders are still to meet on their journeys between floors started at the same time, as in the model? Use the model to help you find an answer or check a calculation.
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Download this Resource
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Package:
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Velocity.exe (496Kb)
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Contents:
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velocity.mdl, velocity.doc and velocity.bmp
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Instructions:
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Download the self extracting ZIP archive. Locate the file using Windows Explorer and double click. The self extractor will then start. The default installation path is C:\AP Revision. Having set the path press the Unzip button. After the files have been extracted change directory to C:\AP Revision. To start the model double click on velocity.mdl.
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