Lets think about the simple mobile I have drawn below:
Two balls are hanging from strings attached to the ends of a stick,
and the stick is itself hanging from the string at the top. This mobile
is balanced by moving the top string left or right on the stick until
the whole thing hangs with the stick level as shown.
Notice that the top string is closer to the side of the stick
supporting the heavy ball that the lighter side. If the big ball were
even heavier, the balance point would be even further to the left, and
if the balls had the same weight the balance point would be in the
middle. As long as the stick is light, it turns out that if you
multiply the weight of the ball by the distance from the support to the
string holding the ball, you will get the same result for both balls.
This is the condition for balance.
In any case, when you are building a mobile the easiest thing to do
is just to move the supporting string around until you find the balance
Now, suppose you want to make the mobile bigger, like in the picture below.
Just work your way from the bottom. First balance the green branch,
then balance the red branch. Last but not least attach the red and
greed support strings to the top stick and balance it. You can make any
sized mobile this way, just do it a piece at a time, bottom up.
(republished on 07/11/06)