What happens when a hammer hits a nail? Do hammer electrons hit nail electrons? If not, what drives the nail in?
Eric Kvaalen
Les Essarts-le-Roi, France
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In a sense, that is true. The electrons of the hammer repel the electrons of the nail, pushing them down a bit. That causes the nuclei in the nail to move downwards too, attracted by the nail electrons. Although the electrical effect travels down the nail at nearly the speed of light, that doesn鈥檛 move the atoms much. It is only when a wave of motion, travelling at the speed of sound in metal, reaches any given point in the nail that the atoms there begin to move appreciably.
Paul Dormer
Guildford, Surrey, UK
It is wrong to think of electrons as being hard balls bouncing off each other. An electron is a very small packet of negative charge surrounded by an electromagnetic field. As such, electrons repel each other long before they make contact. It is similar to pushing the like poles of two magnets together. The closer two electrons are to each other, the greater the force pushing them apart and they never make contact. It takes something of the energy of a particle accelerator to get two electrons to actually collide.
About 50 years ago, a colleague wondered what happens if a fly crashes into a speeding train. Obviously the fly is squashed, but, he reckoned, at the moment of contact, both the fly and the train must be stationary. Again, electromagnetic forces prevent the atoms of the fly from making contact with the train.
Brian Spence
North Berwick, East Lothian, UK
Like many a parent, I used to embrace my children with a fatherly hug as a form of greeting after a period of absence.
This practice ceased a number of years ago after realising the answer to this reader鈥檚 question. We now fist bump and simultaneously greet each other with the words 鈥�10 to the minus eight鈥� 鈥� this being roughly the closest our fists will have got to each other, in metres, before the electromagnetic force repels the parent from the child.
So no, the electrons in the hammer don鈥檛 touch those in the nail 鈥� the momentum of the hammer is transferred to the nail by the electromagnetic force.
In exactly the same way, when we walk barefoot along a sandy beach, we aren鈥檛 touching the sand but hovering just slightly above. Touch is an illusion created in our brain. This, of course, is the classical physics answer. I look forward to hearing the quantum answer: presumably it will require both hammer and nail to be present at the same time, and depend on whether the hammer owner has fed the cat.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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