This might be best determined by experiment. While natural rubber is slightly denser than water and will sink, there will be some small amount of air trapped in a rubber band ball, reducing the density to some extent. Attempting to model and calculate the air content seems somewhat impractical without experimental data, and if you are going to experiment anyway, why not just find the answer directly?
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If they float in water, then it is because the density of a ping pong ball is lighter than the density of water. The density of water is just below 1, and the density of a ping pong ball is much less than 1.
rubber ball
Put an exact amount of water in a measuring cup. Drop in a golf ball. The increase in water level equals the volume of the golf ball, assuming the ball doesn't float. For example, 200mL before ball. 280ml after ball dropped in. Therefore volume equals 40ml or 40 Cubic Centimeters.
you want a soft rubber band ball with strechy rubber bands,but you also want to be carefull that you dont make it to big and to small
As long as it can displace a volume of water equal to its mass (ie its density is less than that of the water) it will float. Most paper will have a density less than that of water (or can be so folded) so will float.