Stopping Road Cases in a Recording Studio Hallway

Studio Notes: The Case That Hit the Door

1The Harbor Lane Community Recording Studio lets local bands and choirs record for free on weekends. Instruments and amplifiers travel in road cases, sturdy wooden boxes on four small wheels called casters. The cases roll from the loading door down a long, smooth hallway to the live room, where musicians play and sing into the microphones. Last month, a loaded keyboard case got away from a volunteer, rolled past the live room and hit the glass door of the control room hard enough to crack it. Studio manager Ms. Oluwaseun Hartigan asked the studio's teen crew, led by Matteo Kristiansen and Yara Bellweather, to find out what it takes to stop a rolling case. Both are eighth graders who joined the studio's program to learn sound engineering, and both had pushed plenty of heavy cases down that hallway.

2To work safely, the crew used one empty case and loaded it with sandbags instead of real gear, and the hallway was cleared during every test. Nobody rode on a case, and crew members walked beside it, never ran. First, the crew measured friction. A crew member pushed the case slowly along the hallway with a force gauge, keeping the case at a slow, steady speed. Then the gauge reading matched the friction from the casters and the floor (Table 1).

3Next, the crew planned the stopping runs. In each run, one member pushed the case so that it rolled past a tape line at 1.2 m/s, checked on slow-motion video against marks on the wall. As the case crossed the line, a second member, walking alongside and holding a strap clipped to the back of the case, pulled backward with a steady force read on a force gauge. The crew found the time from the tape line until the case stopped, using the time stamps on the video. They changed the mass of the case with sandbags and changed the pull on the strap, one at a time, and repeated each run three times. Table 2 shows the averages.

4The control room door is 1.5 m past the tape line. Yara pointed out that while a case slows down at a steady rate, its average speed is half its starting speed, so a case that starts at 1.2 m/s rolls 0.6 m for each second it takes to stop.