# Pendulum - basic physics

Hi all,

As I get further into using Kangaroo as a powerful tool to guide my interest in kinetic art, I’m feeling increasing aware of the many holes in my understanding of how it works. @DanielPiker did a great job explaining here, and I’ve looked at prior pendulum examples - but after doing some very basic experiments, I can’t explain the results.

First, I set up two simple pendulums (point mass at bottom, massless string - “plumb bobs”). I set one to twice the length of the other, and compared their respective periods:

Since the equation describing the period of an idealized simple pendulum is T = 2Pi*sqrt(length / g-force), doubling the length should increase the period by sqrt(2), which is what I found (given more time, the number of 1-length cycles divided by 2-length cycles converges nicely to 1.414).
pendulum length.gh (22.9 KB)

Second, instead of doubling the length, I doubled the mass of the second pendulum (or at least I thought I did), by adding a second point very close to the “bob”. I expected the added mass to cause no (or very little) change to the period, but:

pendulum mass.gh (24.0 KB)

Clearly, I’m missing something - l’il help, please.

1 Like

I have no idea

But, playing with the Fv input and W (Force Vector and Scalar Weighting) on the Load goal has an effect. Increasing Fv (i.e. to (0,0,-2) ) slows down the pendulum, and decreasing W also slows down the pendulum but doubling Fv and halving W does not seem to equate to doubling the mass so you will have to wait until Daniel explains it!

pendulum mass.gh (33.6 KB)

It looks to me like doubling either the vector length or the weighting end up doing the same thing - doubling the gravitational force. Since g is in the denominator of the sqrt() of the pendulum equation, doubling it should lead to a period of .707 * that of pendulum 1. And the cycle-counters confirm this. What isn’t clear to me is how the inertial aspect of mass comes into play.

my guess
numerical solver errors