The Golden Cylinder

You are shown two cylinders – one of them is made of gold and hollow, the other one is made of alloy and solid. Both of them have the same weight, shape, and are colored in black. How can you figure out which cylinder is the golden one, without scratching the paint?

Simply roll the two cylinders on the ground. The hollow one will roll farther than the solid one since it will have a higher moment of inertia.

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  1. When you tap on both cylinders, sound you hear will be different. Hollow cylinder will have louder sound because sound waves can travel easily in the air without any obstacle. Solid cylinder, sound waves are absorbed by the metal filling. That’s why, we’ll hear low pitch sound.

  2. Or you could just, you know, whack a cylinder and see if it’s hollow. Interested to see though, if some of the people that are more educated in this matter than I am could respond and let me know if I’m wrong, if the sound heard when flicking metal is only possible because the sound is vibrating out through the other side of the metal into a non-enclosed space, and therefore you would not be able to hear the sound when flicking the cylinder, or if the vibrations themselves make the sounds and echo them out on all sides, including the non-enclosed space, in which case you would be able to hear the sound.