Students explore how to measure their own reaction time based on the free fall time needed to get out of the way of a dropped marble or ball bearing. All that is needed is a tennis ball and a meter stick. The only thing in they need to solve for t out of an equation d = 1/2 a t squared which involves some algebra. For an easier version, try "How Many Seconds Tall Are You?" by Scorton Creek. There you measure for time and calculate for distance.