 |
 |
Pendulum Swings
Name: N/A
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: N/A
Question:
A grandfather clock with a metal pendulum is located on a
person's deck. Can you explain why the clock loses time in the summer and
gains time in the winter.
Replies:
If the grandfather clock mechanism is of the relatively inexpensive
variety, there is no temperature correction for the pendulum. In the
summer when it is warmer, the pendulum expands and is longer. This causes
a slightly longer period, hence the clock runs slow (literally). The
converse is true in the colder winter weather. The pedulum contracts just
a little bit, and has a shorter period.
Precision pendulum clocks have glass enclosed collumns of mercury on the
base of the swinging platform of the pendulum. When the pendulum cable
would expand, lowering the center of mass, the mercury collumn would also
expand, but upward. This upward expansion raises the center of mass just
enough to cancel the lowering of the center of mass due to the cable's
expansion.
l
l
l ---- cable
l
l
l
l
l l l
l l l
l l l ---- mercury collumn
l l l
l---l---l
e tolman
If you're thinking the different temperature changes the length of
the pendulum, and this changes the rate of the clock -- forget it. A
pendulum takes the same amount of time to tick-tock whatever its
length.
I don't think you have done enough fact-collecting yet to theorize.
You clearly suspect the change in temperature is the cause. Test
this. Move the clock indoors and see if the change persists. You
don't have to do it for a full year: just move it inside for a few
days during the summer, when the inside is bound to be cooler, and see
if the clock starts speeding up. Or else look at how fast the clock
goes during the day and during the night on any day, since the night
will typically cooler than the day.
There are certainly a lot of parts in a clock which change shape
and size slightly in response to temperature, and these could affect
the clock's accuracy. But there are other possibilities, too. It
could be humidity, for example, which will change from summer to
winter. Perhaps dew condenses on the clock, and is burned off faster
by the earlier rising sun in the summer. And so on.
Dr. c. Grayce
This sounds like a homework problem. No sane person would keep such a
valuable piece of furniture outside in the weather. Most likely, it would
rust in the spring, and never keep time again.
So that you can answer this question yourself, what determines the period
of a pendulum? Its length? Its mass? Its kinetic energy? What would
temperature changes effect?
Richard Barrans Jr., Ph.D.
The period (or time) of a pendulum swing is related to the mass of the
pendulum and the length of the pendulum. The mass will not change during
the seasons; however, the length does. In winter, the length shortens and
the pendulum swings faster. The opposite occurs in the summer when the
pendulum increases in length and the pendulum takes longer to swing. I
suggest you go to the library and look in a physics book on the laws
governing the period of oscillation of a pendulum (there is an equation that
will tell you in mathematical terms what I just mentioned). Galilao is
probably the most famous scientist who investigated the laws.
Have fun.
Dr. C. Murphy
Click here to return to the Physics Archives
| |
Update: June 2012
|
|
