Can 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 the construction of China’s Three Gorges Dam has actually made days longer by slowing down the Earth’s rotation. Alleluia! Extra time you can use to learn a new language, catch up on work, or better yet, sleep.
This raises a lot of questions. First of all, is this true? Has the dam really lengthened our days? And if yes, why? And 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 even more time?
Angular speed
To answer these questions, we need three basic physics ideas: (1) angular velocity, (2) angular momentum, and (3) moment of inertia. I will explain each of them.
First, here’s a little experiment you can do yourself. If you have a rotating office chair, rotate it with your feet off the floor and your arms tucked in. Now, before you vomit, extend your arms. It slows you down, doesn’t it? Here is a version with a human on a rotating platform:
Video: Rhett Allain
Rotational speed is what we call angular velocityrepresented by the Greek letter omega (ω). Why “angular”? Here’s a two-dimensional example below. Consider this circle to be centered on a xy coordinate plane. I drew a ray (A.) to a point on the circle where it intersects the horizontal axis:
