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Why does a spring weigh more when it is compressed? Erica Plattner, Brian Peterson, Jenni Noe, Chanel Zacha, Bradley Fyke, Ben Moretz Eureka High School, 9th grade

Well, it does not weight much more! Sounds like a trick question to me! The reason is because compressing a spring stores energy in it, and from Einstein's famous:
E = m c^2
we know that an increase in energy is associated with an increase in mass. And weight is always proportional to mass for a fixed gravitational attraction, so there is your answer. The change in mass is equal to the change in energy divided by the speed of light squared, and is thus very small (try calculating it assuming the energy is 1 Joule!)

Arthur Smith

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