This slider adjusts the strength of the downward pull of gravity on the pendulum bob .
At the Earth's surface this pull is 9.8 Newtons for every 1 Kilogram mass.
On the Moon it is just 1/6^th of this.
The size of this downward force (represented by the red arrow pointing down from the pendulum bob) depends both on the strength of gravity and the mass of the pendulum.

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