You have a nail set in a piece of wood for example. Theoretically if you swung something else at it, no matter what it was, could you drive the nail in, if it were moving fast enough. I get that the object would be destroyed at traveling basically the speed of light in a short distance but if you used a slice of soft white bread or a business card, could it technically work? Again, ignoring how you get it to the speed of light and the integrity of the object while its traveling.

So using the force generated by swinging a business card at the speed of light at the point of impact. Essentially which would fail first? The nail being pushed in or would the card break before the nail could overcome the friction and nail into the board.

I personally think it wouldn't push the nail into the board, but the integrity of the "hammer" would fail before the force could be transferred into the nail.


1 Answer 1


You can't always nail in a nail with any object. On the low speed scales, its obvious that a piece of bread cannot nail the nail in because the bread will deform before the nail overcomes friction and starts to move. Your interest is in the faster cases, where the bread doesn't get the opportunity to deform out of the way.

At high speeds, what you find is that most of the bread will "miss" the nail entierly. There wont be time to transfer its momentum into the sections of the bread that is directly above the nail (via molecular bonds stretching), so the bread just tears.

But you talk about "speed of light" stuff, so we're clearly talking at least hypersonic speeds. When you start to get to these exotic speeds, we have to start looking at more than just the piece of bread and the friction holding the nail together. At these speeds, we don't need the hammer to maintain integrity. The momentum of the particles directly above the nail starts to become sufficient to overcome friction. However, these speeds also mean that momentum is sufficient to break the nail apart, making it cease to be a nail. Since you are moving faster than the speed of sound in the steel of the nail, it will form a shockwave rather than a normal compression wave. This could sheer the nail off at any arbitrary weak point, such that the impulse is not fully transmitted to the nail/wood boundary.

So even ignoring the integrity of the hammer, the integrity of the nail is the thing that fails in the extreme case.

So use a hammer. It just works better. And I highly recommend reading XKCD's Relativistic Baseball what-if. If you're talking about flinging bread around at relativistic speeds, there are plenty of other practical considerations to consider beyond "will it nail."

  • $\begingroup$ :O thank you soooo much! That's exactly the kind of thing I was looking for!!! $\endgroup$ Commented Dec 3, 2020 at 19:40

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