Cliff Divers in the Red Bull Cliff Diving World Series free fall 27 meters over the course of 3 seconds, before entering the water at speeds upwards of 85kph. At that height and speed, the surface of the water hits back like concrete, and the athletes must enter perfectly vertical to withstand the force...or face severe injury and possibly the worst belly flop known to man.
Physics of Cliff Diving
When you jump from a cliff, you go into free fall and gravity is the only force acting on your body. You encounter no resistance from friction with the ground as you would when running or skiing, and you suffer negligible air resistance. But gravity is a powerful force. As you fall, it pulls you toward the earth, or in the case of cliff diving, toward the water, at a speed of 32 feet per second per second (9.8 meters per second per second). Because time is a factor in this formula, the longer you fall through space, the faster you go. Free fall acceleration dictates that every second of your descent has an increased velocity over the second before it.
Higher cliffs don't accelerate your fall -- acceleration is constant during free fall. The pull of gravity decreases infinitesimally at higher elevations, but it pulls you with an equal force from the first second of your jump until you decelerate when you hit the water.
The height from which you jump affects is the speed with which you hit the water. If you jump from 10 feet (3 meters), you'll be traveling at 17 miles per hour (27 kilometers per hour) when you reach the water. If you increase your height to 50 feet (15 meters), you'll increase the speed of impact to 38 mph (61 kph). That's just the vertical velocity of your fall through space, the speed you gain from gravity acting alone -- if you add some horizontal velocity, your impact speed increases. John G. Cramer, a physics professor at the University of Washington, explains that from the same height, "[A] diver who gets a running start and develops a significant forward velocity will hit the water with more net speed than a diver who dives straight down without a push off."
Then, when you hit the water, your velocity drops almost instantaneously. In about a second, you go from your maximum speed to zero