Swing and Crack: Kinetic Energy in a Makerspace Impact Tester

Build Notes: Testing Brackets With a Swinging Striker

1Members of the Foundry Street Community Makerspace print plastic brackets on the space's 3D printers to hold up shelves, bike hooks and plant pots. Some printed brackets cracked the first time something bumped into them, and a cracked bracket can drop a heavy pot or a bike onto someone's foot. Members wanted a quick, fair way to compare designs. Mentor Mr. Obadiah Pratt and two eighth graders, Faisal Rahimi and Greta Sundberg, decided to build an impact tester so members could check a bracket design before using it at home.

2The tester is a metal arm that swings on a pivot like a pendulum. At the bottom of its swing, a striker on the end of the arm hits a bracket clamped to a heavy base. Weights bolt onto the striker, so its mass can be 0.5 kg, 1.0 kg, 1.5 kg or 2.0 kg. Releasing the arm from a higher point makes the striker move faster at the bottom. A light gate just before the bracket measures the striker's speed in meters per second (m/s). A clear plastic shield surrounds the tester, everyone wears safety glasses, and only Mr. Pratt releases the arm.

3The energy that the striker brings to the bracket is its kinetic energy, the energy an object has because it is moving. The team calculates it with the formula KE = ½ × m × v², where m is the mass in kilograms (kg) and v is the speed in m/s, which gives the energy in joules (J). The v² part means the speed multiplied by itself.

4The team ran every setting three times on fresh brackets. The light gate readings for each setting agreed within 0.1 m/s, so the team used the averages. First, they swung each striker so that it reached 2.0 m/s at the bottom. Next, they kept the 2.0 kg striker and released it from three different heights. Finally, they tested the makerspace's most common shelf bracket, Design A. It cracked in every test where the striker had 4 J or more of kinetic energy and never cracked in tests with 3 J or less.