The rock has a mass of 1 kg. What is the mass of the measuring stick if it is balanced by a support force at the one-quarter mark? The centre of mass of the measuring stick is at the one-half mark, one-quarter of the stick's length beyond the pivot.
1 kg
The stick is balanced by the 1 kg rock that is one-quarter of its length to the left and by its own weigh which is at the Centre of Mass, one-quarter of its length to the right. Therefore, the mass of the stick must be one kilogram, 1 kg.