Have you wondered how we know the mass of the planet Earth?
Try this activity, using a pendulum and a watch or timer to measure the period of the pendulum - the period is the time it take for one full swing of the pendulum - which can then be used to calculate the mass of the planet! It is possible to do the same thing on any other planet, or moon, or asteroid. This uses Newton's law of gravity, and the fact that the motion of a pendulum is dependent on how massive the planet is to do the calculation.
The accepted mass is 6 x 10^24 kg, by the way!
