Boy Drowns In Swollen Creek

Tuesday, July 11, 2006

"An 8-year-old boy drowned yesterday after he and his two cousins were swept off the banks of a creek, despite the heroic efforts of two men who dove in to rescue the trio. Two heavy rainstorms swelled the normally calm creek by more than half a meter and the strong currents hampered rescue efforts. The first passerby was able to grab the brother and sister. Despite the valiant efforts of the second man, the 8-year-old boy, who weighed 60 pounds, was swept away.".

Here is the high-school physics behind all this; teach this to your children, your friends, fellow-adults who wish to attempt heroic rescues:

The energy of a moving body is given by the formula ½mv2, read that as "half emm vee squared", where m is the mass of the body and v is the velocity.

If you remember nothing else, remember this: Energy is proportional to the square of the velocity. Double the speed, four times the energy. Triple the speed nine times the energy.

If the creek normally flows at a languid 3 Km/hour and after heavy rain it is flowing at 12 Km/hour, like it or not you have four times the velocity, so four-squared, also known as sixteen times the energy. It's not just flowing faster. It has sixteen times as much energy available to sweep you off your feet.

Here's the kicker: The mass of a flowing body of water is proportional to the velocity. If the creek is flowing twice as fast as normal, then twice as much water is available at any given time, so there is twice as much mass. Triple the speed - triple the mass.

So the energy, composed of mass and square of velocity is actually proportional to the cube of the velocity.

If you remember nothing else, remember this: Energy of a flow of water is proportional to the cube of the velocity. Double the speed, eight times the energy. Triple the speed twenty-seven times the energy.

No wait! There's more! If the creek is normally four inches deep, and runs twelve inches deep after a rain storm, that gives it triple the depth. Three times as much water available at any time to sweep you off your feet. If the creek is flowing three times its normal speed (27 times the energy) and is as well three times as deep, think 81 times as much energy.

One 8-year-old-boy might fight another 8-year-old-boy and win.

No 8-year-old-boy can or should hope to beat 81 8-year-old-boys.

Now you know why trucks get swept away trying to cross the creek when the creek is flowing strongly. "It's only twelve inches, we can get through that".

Wrong!

Now you know why police use fire-hoses to knock down unruly crowds.

Take another look at the creek once the floods have died down. Did you ever wonder how those huge boulders came to be there?

Now you know.

Try lifting one of them. Of course you can't. But if they were under water, they'd be lighter by the weight of the volume of water the displace, meaning less work for you to do to move them along.

Just like an 8-year-old-boy who is swept off his feet.

Saturday, December 04, 2010

From ABC News : “Alan Kane was trying to cross a creek at Bajool, south-west of Rockhampton, when his utility was washed away by floodwaters. Police divers and an SES crew found Mr. Kane inside his utility about 150 meters downstream.”.

The force of a creek’s water can easily carry a small truck away. Remember that all the truck has going for it is negligible sliding friction between the type pad and the road surface, and the road surface is NOT dry this time!

The energy of the creek (cube the velocity!) acts against the sides of the wheels exposed to the water.

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