Foucault Pendulum for Advanced Experimenters 8

1. Introduction to plumb bobs
2. Plumb bob accuracy testing rig
3. Introducing the problem for SPP No 3
4. Answers and experiments to the problems raised in Video No 7
Link to icansciencethat
Why don't plumb bobs wobble
• Why Don't Plumb Bobs W...

[@Xsiondu]

I didn't know what you are on about but I'm subscribed because strange topic. I think I'll enjoy this.

[@icansciencethat]

Yeah... the off-center mass of the pendulum is inducing a significant wobble. That really does look like MORE than enough to create issues with a decent Foucault Pendulum experiment. Once the thing is swinging, I expect it won't spin anymore, and then the centrifugal wobble will go away. But to make that go away while it's spinning, you'll need to perfectly balance it. That'd be a tough one. The angle of the roof shouldn't matter, and now you've got the gravity vector aligned with the axis of rotation, but any off-center mass causes a wobble ... just like what happens to your car if your tires aren't balanced. :(

[@icansciencethat]

I should point out the dramatic difference in rotation speeds between what you showed in video 7 and video 8. The easiest way to reduce the centrifugal force is to reduce rotation rate. Since the centrifugal force scales with the square of rotation speed, the increased rotation speed is turning even small eccentricity into a large force.

[@peterleane6807]

@icansciencethat Thanks for the reply,... Yes to all of the above,.... I could never get consistent CCW precession on any pendulum, until I made the plumb bob / pendulum tester. It was sorta like trying to balance your car tyres while driving the car and as you pointed out just a few grams / or mm out of round on a car tyre will shake a car.

[@iliketurtles4463]

Make the string long enough to just reach a bucket of water on the floor... This is how we used to stabilize our plumb bobs on windy days. Also mass of the bob, how dynamic the string is are among many factors that will affect stability too.

[@larryscott3982]

Not clear on why you have a motor at all.

[@peterleane6807]

By spinning the plumb bob it is more easy to pick up imperfections such as weight, bearing and aglet misalignment. When we start to swing the plumb bob to become a pendulum it has to be as close to perfection as possible, otherwise it will produce inconsistent results. You have to think like a pendulum clock maker trying to make the perfect clock, each swing of the pendulum must be identical otherwise the clock will gain or loose time. Good question I should have amplified the point of spinning and precision.

[@larryscott3982]

@@peterleane6807 So the motor is just to screen the plumb bob that will be used.

[@HotelPapa100]

@@peterleane6807 By testing the bob spinning you over-emphasize the problems of having a bob with an unstable axis of rotation. A lengthy bob will ALWAYS be unstable in rotation.

[@peterleane6807]

@HotelPapa100. The plumb bob accuracy testing rig is only used to look for imperfections in plumb bobs, in other words a screening tool. When the plumb bob swings, (pendulum) I do not spin the motor. That being said in a future video I will be spinning the motor a very slow rate similar to the angular velocity of the Earth.

[@HotelPapa100]

@@peterleane6807 But a bob that has ANY overextension in one axis will be unstable. Independent of its own accuracy. The rotation test is not useful to assess that accuracy. Rotation about its unstable axis will exaggerate any external influence,and you won't be able to eleiminate those completely.