Could a hydroelectric dam really make the days longer?
how many times Have you said, “I wish the days were longer”? Well, NASA scientists say that building China’s Three Gorges Dam actually made the days longer by slowing the Earth’s rotation. Hallelujah! Extra time you could use to learn a new language, catch up on work, or even better, sleep.
This raises many questions. First of all, is it true? Did the dam really make our days longer? And if so, why? And just how much extra time are we talking about here? I mean, you have to plan your schedule, right? Should we build more giant dams to buy more time?
Angular velocity
To answer these questions, we need three basic ideas in physics: (1) angular velocity, (2) angular momentum, and (3) moment of inertia. I will explain each of these.
First, here’s a little experiment you can do yourself. If you have a swivel desk chair, rotate it so that your feet are off the floor and your arms are in it. Now, before you throw, extend your arms. It slows you down, right? Here’s a version with a human on a rotating platform:
Video: Rhett Allen
Spin speed is what we call it Angular velocitywith the Greek letter omega (ω). Why “angular”? Below is a two-dimensional example. Consider this circle at the center of one xy coordinate plane I drew a radius (R) to the point of the circle where it intersects the horizontal axis:
