# How fast the Earth rotates

Discussion in 'Physics' started by physicsstudent1, May 16, 2011.

1. ### physicsstudent1 Thread Starter New Member

Apr 11, 2011
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Hi everyone

I just read an article that explained why the Earth rotates and the speed at which it rotates. The Earth, at the equator, rotates at 1,070 mph!!!!!!!! THAT is amazing. However, I was confused about how they explained the calculation for other lines of latitude since at different latitudes the Earth rotates at different speeds due to its shape. It said "Speed decreases by the cosine of your latitude so that at a latitude of 45 degrees, cos(45) = .707 and the speed is .707 x 1670 = 1180 kilometers/hr. You can use this formula to find the speed of rotation at any latitude." Where did the 1670 come from? Maybe they converted mph to km/h but 1070mph ≠ 1670km/h. I'd like clarification on how to use this formula because I presume that you find the product of cosθ and circumference of Earth to find speed at latitude θ.

Here is the article

http://starryskies.com/articles/2007/11/earth-speed.html

ANOTHER question:

When cruising in an automobile, if i were to throw a ball in the air it should go back right? I've tried something similar but the effect was hardly noticeable. So i was wandering, if you were cruising in a plane and you threw something in the air... shouldn't it stream backwards really fast? This perplexes me... because the moment anything catches air after being tossed from a moving vehicle, it's gone. Now, on a much larger scale... the Earth is moving at 1070 mph... at what point above the earth's surface can you notice this speed. How far would I have to throw something in the air before it just shoots back like a bullet?

Last edited: May 16, 2011
2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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It's all about this little thing called Newtons Law. An object in motion(speed of the earth rotating) tends to stay in motion(speed of the earth rotating).

Since you and the ball are BOTH moving at the same speed(while seeming to stand still on the surface of the earth) Why should the ball speed away from you? Why not YOU speed off at 1000mph and the ball just falls and bounces on the ground? Seem a little ridiculous?

3. ### physicsstudent1 Thread Starter New Member

Apr 11, 2011
7
0
Let me try to explain it this way... If you are on a carousel or merry go round and you try to jump HIGH in the air while it is going around... what happens? Do you land perfectly back on the position you started? or do you think you may have landed a little distance behind where you started... The carousel also moves very slowly but that wouldn't stop you from being displaced when you land on the ground. I am asking, for the response you tried to give, AT WHAT POINT above the Earth or how high is high enough for something not to be part of the motion on Earth so that it is left behind rapidly.

Mar 24, 2008
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5. ### magnet18 Senior Member

Dec 22, 2010
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Theoretically, you could jump halfway to the moon and land in the same place you jumped from, the only reason this would change is because of any wind. Speed, like voltage, is relative. Don't confuse air resistance or drag with the force of the earth rodating.

Tink about it this way, what if you were ina plane going west versus east and drop a ball from both, would anything different happen?

6. ### BillO Distinguished Member

Nov 24, 2008
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I'm not too sure this will work. Can you prove it?

7. ### Wendy Moderator

Mar 24, 2008
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I find it suspect too, given that is 230,000 miles (380KM) away. Technically this is an orbit, even if it is (in theory) straight up and down. LEO is 60 miles up, and geosynchronous orbit is 26,199 miles up.

The speed to circle the earth that high is considerably greater than the speed of earths rotation. In other words, the earth will rotate under you.

With a space elevator 200 miles up you can go into stable low earth orbit simply by letting go at the top of the elevator. The speed of rotation at that height (where it is equivalent to geosynchronous) is much, much faster than the surface of the earth.

8. ### magnet18 Senior Member

Dec 22, 2010
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ooh, you're correct, I was just thinking up and down on a moving object
Didn't factor rotational acceleration.

Going back to the op's question about the plane, and going by what you guys said, if the earth had no atmosphere and you dropped a ball from halfway to the moon, and you were geosynchronous, would the ball would actually move out in front of you as it got closer since it was moving faster?

Last edited: May 17, 2011