Why don't pendulums work at sea?
It appears that what you want to ask is "Why don't pendulum clocks work at sea?"
A reliable chronometer is essential to measure the longitude of a location. While pendulum clocks work very well in land they fail miserably in ships making it difficult for the navigators to measure the longitude of a location. This problem is called " Longitude Problem" and attempts to solve this problem led to the construction of reliable and accurate pocket clocks by John Harrison.
The difficulty with pendulum clocks is because they were constructed based on the observation that - when subjected only to gravitational acceleration , the pendulum oscillate simple harmonically (only for small oscillations) with a characteristic time period. So, to be used as a clock, pendulums must be insulated from other sources of acceleration such as inertial forces. But on ocean-bound ships this condition is not guaranteed because the ship gets rocked constantly by the waves. That is what is meant by "pendulum clocks don't work at sea".
More information about this is available at the following sites:
- Royal Museum Greenwich : http://www.rmg.co.uk/explore/astronomy-and-time/time-facts/harrison
- Dava Sobel's Book on the Longitude Problem :