Q & A: Oscillating and reciprocating motion

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Most recent answer: 10/22/2007
Q:
What is the difference between oscillating and reciprocating motion?
- John (age 15)
Lenzie Acadamy, Glasgow
A:
It's mostly just a matter of words here, although physicists tend to use the words in different contexts so I'll try to explain.

Oscillation, repeating back-and-forth motion, is very common in nature. Often we think of "Simple Harmonic Oscillation" as a motion which varies sinusoidally with time:

x = A*cos(omega*t)

for example, where omega and A are just constants, and x is some variable, such as the position of an object which is oscillating. Many, many systems oscillate in this way, from a plucked string to a ringing bell, to radio waves (x is an electric field, say), and lots of other stuff. Even things that don't move back and forth can oscillate. Something that gets bigger and smaller (with x being the radius of a sphere) can be said to oscillate.

"Reciprocation" is most often used to refer to motion that is repetitive and involves some kind of back-and-forth changing of the position. I wouldn't demand that the position vary sinusoidally with time to say that it is reciprocating (although that is nearly the case with common objects in reciprocating motion). Examples include pistons in pumps and steam engines on trains as things that reciprocate. Reciprocating saws have a straight saw blade that is driven up and down with a motor.

It's also true that oscillators don't have to move sinusoidally either -- their motion can be described with a sum of sine functions, or be described as "anharmonic" if it's truly random. Physicists worry more about oscillating systems of all types and use that word much more often. You'll find the word "reciprocating" a lot more when you talk with engineers and people who design things like pumps and saws.

Tom

p.s. From what Tom says, it sounds like 'oscillating' may usually be used to refer to things which go back and forth many times on their own, like a pendulum or a twangy spring. Even when these things are driven a little bit, the oscillation rate is mainly determined by the oscillator. 'Reciprocating' may be used more to refer to things that are driven back and forth at some rate set more by the driving force, like the piston in a steam engine.

Mike W.

(published on 10/22/2007)