When you push on one end of a long metal rod, the other end of it will move right along with it (unless it is somehow prevented from moving, in which case the rod will get smashed together, if you push hard enough).
But how fast does the other end of the rod move? When first thinking about it, it seems like the opposite end of a rod would move instantly along with the end that is pushed.
That, however, is impossible as it would make faster than light communication possible (if you had long enough rods).
To figure this out, you need to stop thinking of a metal rod as a single piece of material and start looking at it for what it is, an object made up of millions (or more) atoms. The crystalline structure is held in place by the bonds, making them very hard.
So, when you push on one end, you are actually swishing those bonds together on a microscopic level. The first atoms are pushed by you, then they push on the next set of atoms, and those push on the next ones. This is repeated all the way through the metal rod until finally the end moves.
For a solid object like a metal rod, how fast this occurs will depend on a number of factors. Perhaps unintuitively, it all comes down to the speed of sound within that object.
When going through the air, sound moves at about 340 meters (1115 feet) per second. When going through water, it speeds up to 1500 meters (4921 feet) per second. As sound moves through more solid objects, it continues to get faster. The actual speed is also influenced by temperature, pressure, and other factors as well.
So, the answer to the question of how long it takes for the opposite end of a hard rod to move will depend on how fast sound can move through it.
Still don’t believe it? You can actually see this proven using relatively simple household experiments. Check out this video to see exactly how it works:
I would have never guessed!
If you thought that was interesting, you might like to read a story that reveals Earth’s priciest precious metal isn’t gold or platinum and costs over $10,000 an ounce!