Excel has a very important function called goal seek and in this screencast we are going
to show you how to use it. What we are going to use as an example are two children on a
see-saw and one is heavier than the other and what you need to find is how far should
the heavier kid sit from the balance point so that he is perfectly balanced with his
friend. In this particular screencast you can assume that the weight of the see-saw
is negligible. Here we have that the blue kid has a mass of 54 kg and the yellow has
a mass of 45 kg. If you notice here at this point in our initial analysis both of them
are equidistant from the center of the see-saw or two meters. However what this does is create
an imbalance in the forces. The blue kid has a force of 1058.4 N while the smaller yellow
kid has a force of 882 N. In order for this to be balanced those two forces have to equal
each other. One way that you could do it is keep changing this distance here because it
is part of our formula for force where the force is the mass times gravity times the
distance. What we could do is keep changing this until we get these two forces equal to
each other. There is an easier way to do that and that is what is known as goal seek. If
we go into our "what if analysis" and we look at "goal seek". If you notice here goal seek
has 3 inputs. Here what we want to do is set this cell here F16 to the value that the yellow
kid has for force which is 882. What cell are we going to change? The cell that we want
to change is the distance so this is the distance from the center that the blue kid would have
to sit to balance out. Then all you do is click "ok". If you notice both of the forces
are the same. The distance here has changed to 1.7 meters. Now let's cancel this and look
at a different scenario. Why did we choose the blue rather than the yellow. Let's take
a look and instead what we are going to do here is set this cell equal to the value of 1058.4
by changing the yellow distance. We hit ok and the problem with this is if you look at
the distance it is 2.4 meters from the center which means that the yellow kid is off of
the see-saw. Let's go back one more time and show you how to do it correctly. Go to goal
seek. We want to set this cell equal to the value of 882 N by changing the distance from
the center. Now we have both this and this force equal.