What is the kinetic energy of one cubic foot of water traveling at 5 mph?

Answer 1

You must first determine the mass of the water, and you must also do some unit conversion. Since you know the volume of water, you must first determine its mass. One cubic foot of water weighs about 62.4 pounds (pound-force, lbf). But that's weight, not mass. What is the mass of 62.4 lbf of water? Since m = W/g, we get 62.3/32.2 = 1.93 slugs. (Note that the density of water varies with temperature, so the weight of a cubic foot of water will vary somewhat with temperature. It is densest at four degrees Celsius, but I chose to use the density of water at room temperature -- about 20 degrees C.) Miles per hour (mi/hr) is not a convenient unit, so convert to feet per second. One mi/hr = 1.47 ft/s, so 5 mi/hr = 7.33 ft/s. Kinetic Energy, Ek, is proportional to mass and the square of the velocity. It is given by the formula Ek = (1/2)mv2. Ek = (1/2)(1.93)(7.33)2 = 51.85 ft-lb (foot-pounds) = 70.3 joules.