# Determining Center of mass via string method on odd objects

This question is similar to Experiment to determine the center of mass but the answer does not fit my current experiment.

My experiment is that I need to find the center of mass of an odd object like a normal water bottle e.g water bottle.

I was thinking of using a string to tie around the circumference of the bottle, however there isn't a good grip around the bottle, and the bottle keeps spinning.

• The bottle needs to be either empty or full. Jul 12 at 13:38

You can determine the center of mass of your example bottle using one string method experiment plus one logical assertion.

The logical assertion is that the bottle is rotationally symmetric, meaning that its center of gravity must lie on the symmetry center line.

For the string method experiment, make sure that the string is not attached at the bottle's center line (actually, it's hard to attach it there unless you drill a hole) and you don't use too stiff or too heavy a string.

Take a photograph, extend the vertical line given by the hanging string to the point where it hits the center of the bottle, and there's the center of gravity. On the image, you should also verify that there's no bending near the knot, indicating string stiffness.

• This assumes the air volume is negligible. Also, have the centerline of the bottle (its axis of symmetry) point to the right or left in the picture (not towards or away from you). Jul 12 at 14:02
• Dear Ralf, thanks for the comment. May I ask how would you advise on how to execute the string method experiment? I agree that it is hard to attach it unless i drill a hole. However, I am looking for a non destructive method about my objects, and tying a loop or a deadknot about the objects doesnt seem like the best approach due to the spinning. Jul 13 at 1:59
• Use a loop around the bottle's neck. You probably have to experiment a bit with different threads, to minimize the spinning. Take the photograph (or a series of them) when the spinning eventually slowed down. Jul 13 at 11:58
• @JimClark In principle, you're right about neglecting the air volume. But I doubt the string method can achieve an accuracy where the air mass matters. And if the object consists of only one material (same density), the location of the center of gravity doesn't change. Jul 13 at 12:04
• Hi Ralf can I ask you, if it would be more accurate in finding my center or mass when I tie a tight loop around my object or would it be better to tie a loose loop (but slippage will not occur) around my object? Jul 21 at 3:43