Tag: centripetal acceleration

Space station gives physics a boost

By Phil Plait | November 10, 2011 6:29 am

This is one of the coolest videos I’ve seen in a while: during a routine reboost of the International Space Station to a higher orbit, the astronauts on board show that the station tries to leave them behind!

What a fantastic example of Newtons’s First law: an object in motion tends to stay in motion unless acted upon by an outside force. As the ISS circles the Earth, all the forces on it are balanced. You can think of it this way: the force of gravity pulling it toward the Earth is balanced by the centrifugal force (or the centripetal acceleration, which is equivalent*) outward. Because there are no leftover forces on the ISS, it feels like it’s in free fall, what some people call weightlessness. No force means no acceleration which means no weight.

However, that’s not always the case. Even a few hundred kilometers up, there’s air. It’s thin, but over time it robs energy from the ISS, dropping it lower in its orbit. This is called drag, and it’s a very tiny force (too small to feel on board the ISS), but it adds up over time. To prevent the station from falling too far and burning up, every now and again low thrust rockets are used to push it up into a higher orbit.

But that applies a force that is not balanced! Read More

CATEGORIZED UNDER: Cool stuff, NASA, Science

Spinning white dwarf bombs may lurk the galaxy

By Phil Plait | September 20, 2011 6:30 am

Some new research just released asks a question near and dear to me: are there thousands of spinning white dwarfs in our galaxy, just waiting to explode as they gradually slow their rotation?

The answer is very probably yes. Let me be clear, as I always must be when covering topics like this: we’re not in any real danger from these things. Space is vast, and supernovae are few. If these things were that volatile we wouldn’t be here to talk about them in the first place.

But it’s still a very cool scientific question, and actually a fairly simple concept. Here’s how it works.

Imagine a binary system of two stars like the Sun, orbiting each other. One star nears the end of its life, swells up into a red giant, and blows off its outer layers. After a few millions years, all that’s left is its core: a dense, hot ball called a white dwarf. The size of the Earth but with the mass of a star, white dwarfs are pretty weird. They have incredibly strong gravity, which wants to crush them down even further, but they are supported by the electric repulsion of electrons, which is a pretty mighty force. It’s an uneasy truce.

It’s made even uneasier by the other star. It too eventually swells up, and can start to dump matter onto the dwarf (like in the picture above). If enough mass piles up, the immense gravity of the dwarf can induce nuclear fusion. Sometimes the material explodes, flaring in brightness, and we get a nova. Other times, if enough matter piles up — making the total mass of the white dwarf a bit more than 1.4 times that of the Sun — the ignition of fusion can cause a runaway reaction in the star, disrupting it entirely. The white dwarf tears itself apart, and you get one of the biggest and most violent explosions in the Universe: a supernova.

But there’s a hitch. Read More

CATEGORIZED UNDER: Astronomy, DeathfromtheSkies!

You spin me right round baby right round

By Phil Plait | October 4, 2010 7:00 am

New Scientist is reporting that scientists have created the fastest spinning object ever: a fleck of graphene spun up to an incredible rate of a million rotations per second!

Normally, while very cool, that’s not the sort of thing I’d write about here. But I had an idle moment, and wondered about what that rotation really meant. I did a little math, and came up with some astonishing numbers.

grapheneFirst off, graphene is a flat sheet made up of carbon atoms; each atom connects to the others in a hexagon pattern, and the sheet is only one atom thick! This sheet is incredibly strong, and scientists are excited by it because if it can be produced on a large scale it would have tremendous use.

In this case, the scientists created tiny flakes of it only a micron (one-millionth of a meter) across; that’s about 1/50th the width of a human hair. They suspended the flakes in a chamber using electric fields, then spun them up using a beam of light.

I started picturing what that must be like, these tiny whirling motes of carbon, and realized that the forces on a spinning flake must be huge. And by huge, I mean monstrous. When you spin, you feel a force called the centripetal force. It’s what you feel when you’re on a merry-go-round, or a car making a turn (it’s the same thing as centrifugal force, just seen in a different way). The magnitude of this force, how strong it is, depends on how fast you’re moving, and how big a circle you’re making.

I decided to calculate the size of the force on a flake. Read More


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