Write an article based on this "Choose a datum. Measure the datum's distance from the center of the main object as well as from the two additional weights."
The datum is an arbitrary starting point placed on one end of the see-saw. You can place the datum on one end of the see-saw or the other. Let's say the see-saw is 16 feet long. Let's place the datum on the left side of the see-saw, close to the first child. Let's say the children are each sitting 1 foot away from each end of the see-saw. The center of the see-saw is the midpoint of the see-saw, or at 8 feet, since 16 feet divided by 2 is 8. Here are the distances from the center of the main object and the two additional weights form the datum:  Center of see-saw = 8 feet away from datum. Child 1 = 1 foot away from datum Child 2 = 15 feet away from datum