Stories tagged angular momentum


Chilean quake sped up Earth's rotation, tipped planet's axis

Earth spins faster
Earth spins fasterCourtesy NASA
If you read the post about how earthquakes differ, you would know that in the Chile earthquake, a large amount of the Earth's crust plunged under its neighboring crust, bringing it closer to the center of the earth.
Just as Olympic figure skaters spin faster when their arms move closer to their body, the Earth is now spinning faster making our day about 1.26 microseconds shorter than it was before the quake.
Earth was also slightly tipped off balance, like when a spinning skater brings in one arm but not the other. The planet's axis tilted about 8 centimeters. This is insignificant compared to other wobbles measuring several meters resulting from winds and ocean currents.


Yer outta here!: Physics play a big role in the National Pastime.
Yer outta here!: Physics play a big role in the National Pastime.Courtesy Mark Ryan
With the baseball play-offs and World Series coming up, I’m sure lots of folks out there (especially in Chicago and not so much in Minnesota) are agonizing over the question: What is more effective, sliding into base head-first or feet-first?

Well, as usual, science has solved the problem. Using physics and mathematics, David A. Peters, an engineer from Washington University in St. Louis, has figured out which of the two ways is more advantageous. Peters is a huge baseball fan, and a mechanical engineer to boot. He explains it this way:

"There's momentum— mass of the body times how fast the player is moving. There's angular momentum (mass movement of inertia times the rotational rate). If it's feet-first and you're starting to slide, your feet are going out from you and you're rotating clockwise; if it's head-first, as your hands go down, you're rotating counterclockwise. On top of this is Newton's Law: Force is mass times acceleration. Then moments of inertia times your angular acceleration."

So which method gives ballplayers a better chance of making it safely to the bag? Center of gravity seems to be the key.

"It turns out your center of gravity is where the momentum is. This is found half way from the tips of your fingers to the tips of your toes. In the headfirst slide, the center of gravity is lower than halfway between your feet and hands, so your feet don't get there as fast. It's faster head-first."

Regardless of the science, Dr. Peters figures preference for one way or the other among ballplayers is about 50/50. And the whole argument goes out the window when talking about first base. Usually, players are much better off running through first rather than sliding into it at all.

"Mathematically, you might think there's an advantage, but leaving your feet is actually a detriment because you're no longer pulsing (pumping your legs) and you start to decelerate," he says. "When you're running, your get your feet out in front of the center of gravity, so you're getting maybe three or four steps of an advantage."

Dr. Peters was also involved in a previous baseball study covered by the Buzz back in July.

Washington University story