From potential energy to kinetic energy

From potential energy to kinetic energy

And when water is moving at 30 or 40 miles an hour, like the tsunami that inundated northern Japan, the heaviness of water turns deadly. Imagine 1,700 pounds hitting you at that speed, and each cubic yard of water as another 1,700 pounds bearing down on you. The destructiveness of a tsunami is not just one runaway car, but a fleet of them.

“That’s exactly the analogy to use,” said Philip N Froelich, a professor of oceanography at Florida State University. “And by the time you’re talking about a wall of water that’s 10 meters high, if that wave is two miles long into the ocean, it’s basically like a hundred tanks coming across you. Even though it’s a fluid, it operates like a solid hammer.”

Not quite like speeding cars

Water does not act quite the same way as speeding cars. As a fluid, it can slip around some objects like round columns, while slamming full force when a large wall is in its way.

It also gathers debris — dirt, cars, trees — as it flows. Those added projectiles can create more destruction as they crash into other objects. Even if the wave only comes up to the knees, the force is enough to knock a person down.

The power of a tsunami comes from straightforward physics. An earthquake suddenly pushes part of the sea floor up or down.

That changes the height of the water above it — what physicists call potential energy — and the potential energy quickly changes into the kinetic energy of the tsunami waves.

In a rough guess, Harry Yeh, a professor of ocean engineering at Oregon State University, said that the earthquake pushed a section of sea floor 250 miles long and 50 miles down by an average of one yard. That resulted in billions of cubic yards of water — trillions of pounds — suddenly shifting position.

That energy going into the tsunami, according to Professor Yeh’s estimate, was a bit less than that of an exploding atomic bomb.

Impact on the earth

In addition to the damage that a tsunami can inflict along coastlines in particular countries, it can also have an effect on the entire earth. The planet’s oceans are very heavy, applying enormous pressure to the ocean crust. When the distribution of that pressure is shifted, as it is during an earthquake, it can induce wobbles in the earth’s rotation.

It takes a lot of force just to keep water in place. More than three million cubic yards of concrete hold back the water behind Hoover Dam on the Nevada-Arizona border.
At its base, the pressure of the water reaches 45,000 pounds per square foot. In that case, however, the enormous power of water is put to positive use: to generate electricity.