In technical language, resonance occurs when the force driving an
oscillating object varies at the same frequency, conveniently called
the "resonant frequency", that the object would naturally oscillate at.
To figure out what this means, think about a pendulum swinging in
front of you. Watch it swing for a while, then start giving it little
pushes at random times and in random directions. What will happen?
Sometimes it will swing, other times you'll stop it, but it will never
get very far. Now, instead of pushing randomly, every time it starts a
swing, help it along by pushing it in the same direction. The amplitude
of the swings will get large very quickly. That is because the force
you are applying to the pendulum is in RESONANCE with its natural
The exact same thing happens when you swing on a swing. If you pump
your legs in time with the swing's motion, you achieve resonance, and
you swing higher.
(republished on 07/13/06)