Vertical Impact of a Sphere Falling into Water

The nature of the drag force on an object moving through a fluid is well documented and many experiments have been described to allow students to measure the force. For low speed flows the drag force is proportional to the velocity of the object, while at high flow speeds the drag force is proportional to the velocity squared. The basic physics depends on whether the flow around the object is laminar or turbulent. It is difficult to observe the flow in a student laboratory, although a dye can be injected into the flow of water for demonstration purposes. An alternative method is described in this paper that allows both the drag force and the initial flow pattern to be measured easily. The technique is simply to film an object when it is dropped into a tank of water. The results can be spectacular when filmed in slow motion, adding to the interest in the experiment itself. The results are directly relevant to the problem of calculating the impact force on an object that falls into water. Water is not as hard as concrete, but it can still exert a large force if the object (or a person) impacts at high speed.