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Barn Door

Body in orbit

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Hi, Velocity has two components - speed and direction. If either one of these changes then a body is said to be under going acceleration. So when I throw a ball, it gradually travels towards the ground and gets faster as its being accelerated by gravity and this effects both of the components of its velocity. However, a satelite in orbit contiues at the same speed even though its direction is changing. So why does gravity only change one component of a body's velocity when its in orbit? Or, in other words, why doesn't the shuttle fall faster and faster around the Earth? Cheers, BD.

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Acceleration is the variation in velocity. From what I know, it has nothing to do with direction...

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Guest Anonymous Poster
During circular orbit, acceleration is always directed normal to the velocity vector, ie there is no acceleration in the direction of motion, so the speed will not change, only the direction.

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It continues at the same speed only if the orbit is perfectly circular. Maybe not even then since the earth is not a perfect sphere nor is it's mass perfectly evenly distributed.

Gravity is pulling the satellite toward the earth, however the satellite's inertia is causing it to move away from the earth. These balance out otherwise the satellite wouldn't be in orbit at all.

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Quote:
Original post by Barn Door
Or, in other words, why doesn't the shuttle fall faster and faster around the Earth?


When you place something into orbit, you need to calculate a precise speed and distance from the earth such that gravity keeps you accelerating "downward" just enough to keep you at the same height. So in general, most things would fall to earth or escape into space. If you're asking why the shuttle doesn't speed up as it orbits, that is because all the acceleration from gravity is perpendicular to the forward velocity, so the orbiting speed remains the same.

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Original post by lonesock
When you place something into orbit, you need to calculate a precise speed and distance from the earth such that gravity keeps you accelerating "downward" just enough to keep you at the same height. So in general, most things would fall to earth or escape into space.


Well, there are also elliptic orbits to consider. For example, to stay in a low Earth orbit, you need a tangential velocity of about 8 km/s. The delta-v (that is, velocity change) required to attain an escape trajectory from LEO is about 3 km/s - easier than getting to orbit in the first place, but not easy. Anything less than that and you'll just end up in a more or less elliptic orbit.

Similarly, while re-entering into Earth's atmosphere from LEO is not difficult because you can use aerobraking to generate most of that 8 km/s delta-v,
"falling down" from the geosynchronous orbit, for instance, requires considerable thrust to be applied before you end up in a elliptic orbit with enough eccentrity to intersect Earth's atmosphere (or Earth itself!)

So, you can't just "spiral out of an orbit" if the calculations happen to be a bit off - for example, the Shuttle just doesn't have *anywhere* near the amount of propellant to even attain an orbit higher than some 500km, much less actually escaping Earth altogether.

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Thanks for the replies.

I kinda worked it out myself eventually and my super intuitive explanation is...

Gravity never gets 'behind' the shuttle so gravity never pushes it any faster.

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the Shuttle just doesn't have *anywhere* near the amount of propellant to even attain an orbit higher than some 500km, much less actually escaping Earth altogether.


Do you know how much more powerful SpaceShipOne would've had to have been to attain orbit?

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Original post by Barn Door
Quote:
the Shuttle just doesn't have *anywhere* near the amount of propellant to even attain an orbit higher than some 500km, much less actually escaping Earth altogether.


Do you know how much more powerful SpaceShipOne would've had to have been to attain orbit?


MUCH more powerful, it only burned for like 80 seconds IIRC. I'd say it would have to be ~50 times more powerful to reach orbit. It also only reached ~100km, enough for a short-term orbit, but the atmosphere will drag you down.

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