Re: What is the principle behind a metronome?

The metronome, as you know, is a device that is used to keep time for 
musicians.  Thus it would stand to reason that the metronome would have to 
keep regular time, much like a watch.  The difference is , however, that 
a metronome can be set to the desired beat, or count.  This is accomplished 
by decreasing the length between the pivot or attachment point and the 
weight.  This action makes the metronome swing from side to side faster, 
and increasing the length between the pivot and the weight will make the 
arm  swing slower.  The time it takes for the pendulum, ( think of a 
metronome as a pendulum upside down) to swing from one side to the other 
and back again is called the period.  
	The movement of the pendulum, any pendulum, is called simple harmonic 
motion.  As the metronome (pendulum) swings one way, a magnet (or a wound up 
spring) pulls on the metallic bar and causes it to accelerate toward the 
zero point or center.  At this point the acceleration is zero, and the 
velocity is at the maximum.  As the bar continues to move in that same 
direction the magnet causes a deceleration at the moment the bar passes the 
center.  This force continues until the bar reaches a velocity of zero.  
This would be the point at which the metronome has reached the maximum 
distance away from the center on the opposite side from where we began.  
This action continues as long as the machine operates.  Because the action 
is precise and regular, pendulums are used to keep time in clocks, as well. 
Quartz crystals vibrate at a particular frequency which can be considered 
simple harmonic motion, and are very useful in watches.  Mathematical 
formulas can be obtained from any physics text.  
	Simple experiment.  Tie a weight on the end of a string that is exactly 
20cm long.  (20cm from the center of the weight to the point of 
attachment.) Pull the pedulum 10 cm away from the center and time the 
period. (Remember, the period is the time from one side to the other and 
back again.)  Do this again except with a 10cm length of string.  See what 
happens. ( Keep time even as the pendulum slows down!)

Thanks for the question,

Robert  L. Judge
Holy Cross School, New Orleans