Hammer Ruler Trick (page 2)

based on 22 ratings
Author: Mack Levine
Topics: Fifth Grade, Physics


You should have gotten everything to balance nicely, as long as the hammer’s head was hanging somewhere underneath the table.


Think about kids on a seesaw. The plank on a seesaw is a lot like our ruler! When it has nobody on it, the center of gravity is smack-dab in the middle, right where the fulcrum is located. Now, if two equally heavy children sat at either end of the seesaw, would anything change? Not really. The center of gravity stays the same, because the seesaw’s mass is still equally distributed on either side of the fulcrum.

What happens when a heavier kid sits on one end of the seesaw? The center of gravity is no longer directly above the fulcrum, so more torque, or force, is applied to one side of the seesaw than the other. The weight of the heavier child causes his end of the seesaw to sink to the ground, while the lighter child is lifted up into the air.

This is very similar to what we saw happen with our hammer and our ruler—but instead of applying weight to the top of a seesaw like a child would, the hammer’s center of gravity pulls on the ruler from underneath the table. In fact, as long as the center of gravity (the hammer’s head) stays somewhere underneath the table (and on the table side of the fulcrum), the ruler remains perfectly balanced. If you used a plastic ruler for this experiment, you might see the ruler getting flexed up into the air because of torque being applied through the hammer’s handle!

Now that you’ve learned how center of gravity helps objects balance, what other things can you build that can balance on the edge of a table? Keep what you’ve learned about torque, balance, and center of gravity in mind as you explore.

Add your own comment
Recommended Learning Products
Trust to find smart things kids love
Unlimited Workbooks and Worksheets
90% of Students Understand Concepts Better Since Using PLUS
Make Math Practice Fun and Engaging
Interactive Math Lessons for Elementary School Students