Physics Help Forum Question about orbital speed

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Oct 25th 2017, 11:32 PM #1 Junior Member   Join Date: Oct 2017 Posts: 1 Question about orbital speed Hi guys! New guy here. Anyways, I was thinking about why the moon doesn't fall to earth today and as I did some research, found that the orbital speed is equal to the square root of the gravitational constant multiplied by the mass of the Earth divided by the radius from the center of the earth. So this got me thinking, and I was playing around with my calculator and set a radius of (6,371,000 [radius of Earth] + 5) meters and got an orbital speed of 7907 m/s. Does this imply that if somehow I were able to shoot an object from 5 meters above the surface of the earth at that speed - I would be able to send that object into orbit? And if so, would it be at the same altitude or will the altitude change and how/why? And as a quick follow up question, once this orbital speed is reached, the object technically won't fall back down to earth because inertia right?
Oct 25th 2017, 11:58 PM   #2

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 Originally Posted by AltamashMomin Hi guys! New guy here. Anyways, I was thinking about why the moon doesn't fall to earth today and as I did some research, found that the orbital speed is equal to the square root of the gravitational constant multiplied by the mass of the Earth divided by the radius from the center of the earth. So this got me thinking, and I was playing around with my calculator and set a radius of (6,371,000 [radius of Earth] + 5) meters and got an orbital speed of 7907 m/s. Does this imply that if somehow I were able to shoot an object from 5 meters above the surface of the earth at that speed - I would be able to send that object into orbit? And if so, would it be at the same altitude or will the altitude change and how/why? And as a quick follow up question, once this orbital speed is reached, the object technically won't fall back down to earth because inertia right?
If there were no atmosphere you certainly could "launch" an object into such a low orbit. You might be interested in the so called Newton's cannonball.

It's not inertia that's keeping it from hitting the ground. The object is actually in free-fall. It's just that as the object falls toward the surface the surface is "falling away" beneath it at the same rate. Astronauts are often said to be weightless because gravity appears to have no effect on them. This is not true...they are orbiting just like the cannonball.

Long ages ago I read a short story about the first Mars expedition. The Martians had a long straight trench in the center of one of their towns and told the Earthlings not to stand there. Something came whizzing by and the Earthlings realized it was small moon in very low orbit. So Mars had three moons, not two, named Deimos, Phobos, and (wait for it).... Bottomos.

-Dan
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Last edited by topsquark; Oct 26th 2017 at 12:05 AM.

Oct 27th 2017, 08:26 PM   #3
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 Originally Posted by topsquark If there were no atmosphere you certainly could "launch" an object into such a low orbit. You might be interested in the so called Newton's cannonball.
What's weird about your response is that at almost the same day that you wrote this I wrote a similar response in another forum to almost the exact question. At first I thought you were repeating what I already said (which is quote unlike you) and was confused until I recalled the other forum.

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