Posted on 17 May 2011 at 14:54h

I was bored doing an exam supervision for Spanish A so kept myself awake by trying to think up some new practicals for next year. I don't have many examples of conservation of energy so thought that might be a good area to think about. The idea I came up with was to project an object vertically upwards from a compressed spring, I thought of using a tube with a spring at the bottom and a pencil as the projectile. After the supervision I went to the lab to play around and couldn't find a suitable tube in any of the drawers in the chemistry lab so decided on inverting the idea putting a rod inside the spring. Behind the door in my lab is the top section of a carbon fly rod that was broken when I tried to teach one of our formers maths teachers how to fly fish, once the eyelets were removed this served as a perfect rod so I mounted it in a clamp stand and started experimenting firing a cotton reel up it. After a while I realised that it was very difficult release the cotton reel so that the height was the same for a given compression. However when I let the reel drop onto the spring I could tell by listening if the spring was fully compressed or not, bang or boing. So by increasing the drop height I could measure the minimum height required to fully compress the spring (the point when the boing becomes a bang).

When this occurs mgh = ½kx²

Now if the same spring is used ½kx² is a constant so m ∝ 1/h

I changed the mass of the reel by winding some solder round it then took a quick set of measurements resulting in the following graph. I was quite pleased about, especially the intercept. Will check out if the value for k that I get from the gradient is reasonable or not.

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