# Seesaw Science: The Hammer-Ruler Trick

A fun physics demonstration from Education.com

Observations and Results
Let's refer back to our hypothetical seesaw. Imagine that more of the seesaw's plank were shifted over to one side of the fulcrum, making that side of the seesaw much longer than the other (just like our ruler hanging over the edge of the table). If you place a large enough person on the shorter end of the seesaw, that person's weight will apply more net torque to that end, suspending the lighter end of the seesaw in the air. The head of our hammer performs a very similar function, but instead of pushing down on the shorter end of the seesaw like the weight of a child would, the gravity acting on the hammer's center of mass pulls on the 0-inch tip of the ruler from below the table.

Hopefully, we managed to dissect the illusion in a scientifically revealing way. So why does it fail to make intuitive sense to us on a visual level? The hammer's handle and the ruler both appear "large," and they both hang much farther over the edge of the table than they "should." At first glance, our instinct is to think, "How can that be?" Of course, the answer lies in the fact that the hammer's head is more massive than its lighter wooden handle, which puts its center of mass closer to the head (and subsequently somewhere under the table). But this is rarely the first thing that occurs to us!

Victorian toys take advantage of the same human perceptual tendency. Think about how these toys are constructed and you'll notice that the large, ornamental part of the toy is made up of thin sheets of metal whereas the toy's counterweight, although small and inconspicuous, is heavy.

More to Explore
Center of Gravity, from NASA.gov
Center of Mass, from PhysicsLAB
Hammer Ruler Trick, from Education.com
Center of Gravity of Symmetrical and Asymmetrical Objects, from Education.com
Torque and Position of Center of Mass of an Object, from Education.com

This activity brought to you in partnership with Education.com

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