Balancing a Hammer and a Ruler

1. You will need: • A ruler (lightweight plastic is best.) • A hammer (heavier is better, especially the head. Recommend a rubber handle • Short length of fishing line or string Balancing a Hammer and a Ruler Step one: make a loop with the twine and place it over the handle of the hammer. It is important that the line not slip, so position it where the handle attached or another high friction location. If necessary a small groove can be filed into a wooden handle. Step two: hold the ruler so that only ½ inch or so is on the table. Pass the loop of twine over the ruler so that the head of the hammer is under the table. Step three: rest the end of the hammer firmly against the ruler. Carefully release. If the ruler does not balance, try sliding the hammer farther towards the table. Safety: have a soft object (not toes!) beneath the hammer while first setting it up in case it falls. Do not allow students to touch the set up – it may not be very stable.

2. The principles: There are two main principles at work in this mini-lab. • Center of gravity • An object will balance so long as the center of gravity is over (or under) the point of support. The net force of gravity is always straight down when measured relative to the center of gravity. Since there is a support point (table) directly above this center, the object is held in place. The lighter the ruler and the heavier the hammer head, the farther under the table the center of gravity will be and the farther out from the table you can hang the entire apparatus. • Friction • The entire apparatus must hold its shape. If the parts do not hold together than individual pieces will behave in accordance to their individual centers of gravity. Specifically, the hammer will slide out and then the ruler will fall.