# I Friction in a simple mathematical pendulum

1. Feb 14, 2019 at 3:26 AM

### MathematicalPhysicist

How do I guarantee that that the friction in the movement of a simple mathematical pendulum is negligible?

2. Feb 14, 2019 at 3:38 AM

### BvU

Use a heavy weight, minimize air resistance, use diamonds for bearings, etc...

3. Feb 14, 2019 at 3:58 AM

### MathematicalPhysicist

Heavy weight compared to the stand that the pendulum is situated on it, or something else?

4. Feb 14, 2019 at 5:28 AM

### BvU

the air that has to be pushed aside for the pendulum to move. Can also be achieved by putting the whole thing in vacuo.

To what purpose ? Saving energy, building a pepertuum mobile, verifying $T=2\pi \sqrt{g\over l}$, other ?

Familiar with the Reversible (Kater's) Pendulum ?

5. Feb 14, 2019 at 5:37 AM

### MathematicalPhysicist

Verifying the formula.

6. Feb 14, 2019 at 5:42 AM

### MathematicalPhysicist

The link you gave is for the physical pendulum, I referred to the mathematical point mass pendulum.

I should have said that it's point mass.

7. Feb 14, 2019 at 5:43 AM

### BvU

There's a good expression for $T$ with damping, so you can measure the damping by following the amplitude as a function of time (all pendula have damping) and correct $T$. Depending on the accuracy of your measurements (in particular: L!), you can allow quite a bit of damping before such a correction becomes the main source of inaccuracy.

8. Feb 14, 2019 at 5:45 AM

### BvU

Do they exist ? What are you doing to verify this T formula ?

9. Feb 14, 2019 at 5:46 AM

### BvU

Nice thing about that one is that it is the exact equivalent of a mathematical pendulum.

10. Feb 14, 2019 at 6:00 AM

### BvU

Don't see no point mass ...
So you want to ask yourself:

With what accuracy can I measure L and T
Do I need to correct for damping ? How much damping is there ?
What is the correction to T for the fact that this is a ball and not a point mass ?

11. Feb 14, 2019 at 6:03 AM

### Staff: Mentor

The post has been removed because it contained a picture in which some personal information could be seen.

12. Feb 14, 2019 at 7:41 AM

### BvU

Only looked at the picture when it was still there.

Way I meant it was that a $\ \ \approx$ 1 inch diameter metal ball is not a point mass.

13. Feb 14, 2019 at 7:55 AM

### MathematicalPhysicist

So I guess it should work as a mathematical pendulum when the angle of release of the ball is small.

How do I find the limits of small angles appropriate for a suitable mass?

14. Feb 14, 2019 at 7:57 AM

### BvU

Experiment !

 and of course, nowadays you also use your big brother friend google for a peek at the expression ... $\qquad$

15. Feb 14, 2019 at 8:02 AM

### MathematicalPhysicist

@BvU but can't I know from theory what to check for?

16. Feb 14, 2019 at 8:03 AM

### BvU

was one step ahead of you see #14

17. Feb 14, 2019 at 8:09 AM

### MathematicalPhysicist

Good to know there's big brother.

I feel like there's no more need to work, everything is in the net nowadays :-D

18. Feb 14, 2019 at 8:17 AM

### BvU

Yes, why bother trying to determine $g$ when you can also look it up