Course content > Newton's Laws > Kinds of Forces
Prerequisites
Our next non-touching (action-at-a-distance) force is electricity. Of our three -- gravity, electricity, and magnetism -- it's probably the one with which you have the lest direct physical experience -- except to be careful of it! Everybody knows and feels gravity, and every kindergartener has played with magnets. But we have been told from a very young age to be afraid of and avoid electricity. We are not supposed to try to attract lightning strikes or stick nails or screwdrivers into outlets. Still, in spite of our parent's best efforts we do have some direct experience with electrical forces.
If I get a shirt fresh from the dryer it will often seem "clingy" -- that is, the different parts of it will stick together. Sometimes -- even days later -- I'll find a sock clinging to the inside of a shirt when I go to put it on. One year, when my daughter was just a toddler continually clutching a woolen blanket, we had new nylon carpet put in on the stairs in our townhouse. She came down it the first time dragging her blanket down the stairs and when she touched the doorknob to go into the den, produced a two inch spark, shocking her (literally) to tears. And most people know that if you rub a plastic comb with wool, you can pick up small pieces of paper, as shown in the figure at the right.
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None of this seems very scary or dangerous. Amusing perhaps. The phenomenon was known to the Greeks nearly 2500 years ago. If you rubbed amber (Greek ηλεκτρον = electron) with wool, you got the "sparky" phenomenon we've just been describing. It took 2000 years for people to realize that something extremely important was going on here. First, the comb -- with just a bit of rubbing -- is able to create a force on the bits of paper that is stronger than the entire earth pulling back on it with gravity! This indicates that there is another action at a distance force taking place -- and one that is way stronger than gravity.
Second, where did the "electrical agents" -- whatever they are -- come from? The answer is that matter itself is highly electrical. It consists of extremely powerful counterbalancing electric agents. The forces between them are so strong that electrical forces are the main ingredient that holds matter together, and the rubbing examples above show what happens when these powerful balanced forces hidden inside matter become slightly out of balance, simply because a tiny fraction of the counterbalancing electric agents literally rubbed off from one material to the other.
This is a theme that will recur again and again as we study physics. The materials and objects that seem to us inert and ordinary are instead highly dynamic balances of immense energies and forces. Learning how to produce small shifts in those balances provide us with the powers of modern technology.
Follow-ons
Joe Redish 10/9/11
Wolfgang Losert 9/29/2012
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