Boards on the Loose
1At Wexley Park's skate park, the bowl and the banks are busy every afternoon. When a skater falls, the board often keeps rolling. Runaway boards shoot off the bottom of the long bank, across the flat and into the walkway where parents and younger kids stand to watch. Last summer one hit a parent's ankle hard enough to leave a deep bruise. The parks department asked the eighth-grade physics club at Ashby Middle School to test board stoppers that could be bolted along the edge of the walkway before the busy summer season begins.
2The club started with Newton's third law. When two objects collide, each one pushes on the other. The two forces are the same size, point in opposite directions and act at the same moment. This is true even when only one object was moving before the collision or when one object is far heavier than the other. The law works for pushes without a crash, too: when a skater pushes off, her foot pushes back on the ground, and the ground pushes her foot forward just as hard.
3To test each stopper, club members Nilufar Okoro and Kenji Szilagyi let a 2.0 kg skateboard roll down a test ramp so it reached 4.0 m/s at the bottom and hit the stopper head-on. A force sensor taped to the board's nose recorded the peak push of the stopper on the board, and a video app measured the board's speed after the hit. They ran each test three times with the same board, ramp and starting point, and Table 1 shows the averages. Each stopper was bolted to a concrete floor, as it would be at the park.
4The parks department gave four criteria. First, the board must not get past the stopper. Second, the board bounces back at no more than 1.0 m/s, so it does not roll fast back into the skating area. Third, the peak force between the board and the stopper is no more than 1,500 N, because the bolts that hold a stopper to the concrete can work loose above that. Fourth, the stopper is no taller than 10 cm, so people can step over it without tripping.