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Name: Dan
Status: student
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Question:
How do I calculate the potential energy of a mouse trap spring?



Replies:
Educator, In most cases, there is no easy formula to calculate potential energy. As a great deal of energy is lost to sound, it may not even be conservative. I will use language that assumes you are using a traditional mousetrap. Still, the method is very general.

If you want a precise measurement, then use a device that can measure force, something that can handle whatever force is needed to hold the trap in its set position. Keeping the scale or sensor directed parallel to the path, measure the force needed to hold the trap still at various points along the path. Use distance along the circumference as your distance measure. Be sure to note that zero position is NOT zero force. For zero, use the force needed to barely lift the trap from the wooden base. For safety, be sure the base is clamped in place.

Graph force as a function of position. Calculate the area under the curve. You may even be able to estimate it as a triangle. For your scale use Newtons, NOT kilograms. Many scales are unfortunately marked in kilograms for convenience to grocery workers. Use meters for your distance along the circumference. The area under the curve will be in Joules.

If you want an approximate value, assume that the mousetrap will operate as a spring. The work done to set the spring, and thus the potential energy will depend on two quantities: average force and total distance. Find the force at the start, that which is just enough to open the trap. Find the force that is just enough to hold it in position near the end. Calculate the distance along the circumference, start to finish. 0.5*(F_start+F_end) is your average force. Multiply this by the total distance along the circumference (perhaps pi*radius). Now you have an excellent approximation for the potential energy in a set mousetrap.

Dr. Ken Mellendorf


It is not so easy to calculate, but you can measure the force required to stretch the spring as a function of the displacement by hanging known weights from the "trap" end of the mousetrap and measuring how much the spring is wound up. You will want to take safety precautions in the event a weight should "slip" because you could get a nasty bruise, or even a broken finger if something should slip. You can obtain the potential energy directly from the weight and the displacement of the trap. You can find the formulas on any web site using the search term(s): Hooke's Law potential energy. There are a couple of ways to manipulate the measurements depending upon how you set up the experiment.

Vince Calder



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