See-Saws and Levers:

From early days you know that on a see-saw (or teeter-
totter), a small person can be "small but mighty" as he or 
she raises a parent or older kid, if the heavier person can 
only be persuaded to sit close to the fulcrum and let the 
little person sit far away.  

And levers are used in practical applications to allow a 
small force to move a large object.  

In this demonstration, with pennies and a meter stick, 
you can test this out for yourself.  It turns out that the 
amount of "twist" you can exert depends not only on the 
force but also on the "lever arm" (the distance, in 
perpendicular projection, from the point of application of 
the force to the axis about which the twist is occurring).  

When the see-saw is not moving, we saw it is at the 
balanced condition.  For this condition to obtain, the 
"torques" (or "twists) in different directions cancel out.  
For the simple case of the seesaw, the force is the 
weight of the pennies (mg), and the lever arm at balance 
is just the distance of the objects from the axis.   

Try it out!