Grandfather Clock

[Vinny]

As long as it is I'm thinking I didn't get lost, I got bored!

[Dr Stan]

The weight (mass) of the pendulum does not matter only if it is swinging in a vacuum.

[Vinny]

Or hanging on a clock. Did you read what I posted about it?

[f350ca]

The combination of the shaft and bob make up the pendulum, as they said a heavier bob would lower the center of gravity effectively increasing the length of the combination.

[TomG]

Physics is physics, no matter what the application and the length of a pendulum is what determines its period. The guy in the article is claiming that the weight of the bob also has an influence, but in reality that would only happen if the additional weight changed the effective length of the pendulum by shifting its center of mass, i.e. changing its effective length. That would only happen if the bob was asymmetric, or not mounted in the exact same position on the pendulum. Added weight does change certain characteristics of the swing. A heavier bob for instance would shorten the amplitude of the swing because it requires more energy from the clock, but the period would remain the same because for small swings, the period doesn't change with the amplitude of the swing (isochronism). I knew there was a reason to hold on to those college physics books.

Tom

[Vinny]

On the back of the new bob are two holes for mounting. They are in the EXACT SAME SPOT as the old one. The clock runs considerably slower with the new one - adjustment couldn't fix it.. I put the old one back on and after minor adjustments, it's back to keeping time. The only difference between the two pendulums are the weight, and the new one is shinier. Same diameter, same thickness.

Tell you folks what. Got a mechanical clock with a pendulum? One that actually uses the pendulum, not just for looks. Don't move the pendulum but make it heavier and get back to me. In the mean time, I have no more to say on the subject.

[Hawkeye]

Another way to check the theorem would be to make a pendulum with a disk bolted on the end. Start the swing and a stopwatch at the same time and time, say, ten swings. Then bolt a second disk on at the same location. Time it again. This will tell you if added weight changes the effective length.

[TomG]

The problem with adding weight to a pendulum is how do you do it without changing its center of mass? A few thousandths shift can have a significant effect on the period over time.

Tom

[wawoodman]

Sounds like a perfect example of “it works in practice, but not in theory...”