Science Buddies: "Ask an Expert"

I am helping my son with his project. He has completed the trials and now we are analyzing his data. His data shows that the ball bearing travels farther with each magnet stage added but not at equal increments. After adding the second stage the ball travels 8 more cm than the first stage but by the time he added the fifth stage the ball traveled just 1 more cm than the fourth magnet stage. Does this make sense? If so, why? If not, what should that data look like? Thanks!

Hello and welcome to the forums:

I haven't done this exact experiment, but your results make sense to me. You are looking at the "law of diminishing returns". At some point you reach the limit of adding more energy to the ball bearing.

Keith

kbcat,

Here is an attempt at explaining why. Magnetic fields are additive, and by this fact, one would expect that putting two equal magnets together would double the field strength. However, magnetic field strength drops off exponentially as you move away, so the pull is at its strongest right at the surface, but significantly less so just a little ways away. Now you must consider that the two magnets do not occupy the same space. So, if you have 2 magnets stacked up and you place an object on top, you will feel the full effect of the first magnet, but will be the width of the first magnet distance away from the second magnet. Therefore the pull from the second is slightly less than the pull from the first, but still having an effect. So you have a pull that is more than a single magnet, but less than double. This continues to be the case as you add magnet stages...the third magnet is two magnet widths away, etc...

Additionally, you are still working to overcome drag and gravity forces that at some point adding magnet stages will cease to have a significant counter efect for an overall "diminishing returns" as mentioned by Keith.