Geostationary and polar satellites, and weightlessness · Physics Class 11–12 | StemPy Lab
Course 7.3·Lesson 3 of 3
Geostationary and polar satellites, and weightlessness
A geostationary satellite orbits over the equator, eastward, once per day, so it stays above one place; this needs r≈42300km, a height of about 36000km
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Satellites that stand still
A geostationary satellite circles the Earth once a day, in the plane of the equator, in the direction the Earth turns. So it keeps pace with the ground and hangs above one place: a dish can point at it and never move.
T=24h (strictly 23.93h) in T2=GM4π2r3 gives r≈4.23×107m, a height of about 36000km.
Two drawings of the Earth with an orbit. On the left, a geostationary orbit seen from above the North Pole: a large circle in the plane of the equator, with a satellite on it. The Earth turns from west to east. On the right, a polar orbit seen from the side: a narrow ellipse that goes over the North Pole and the South Pole, about 800 kilometres above the surface.
Polar satellites
A polar satellite crosses over both poles, usually only a few hundred kilometres up, with a period of about 100 minutes. The Earth turns beneath it, so each orbit covers a new strip, and in a day it sees the whole planet. Weather, mapping and remote-sensing satellites use such orbits.
You should find the period crossing 24 hours at about 35900km.
Weightlessness
What you feel as weight is the push of the floor, not gravity itself. In a lift accelerating downward, the floor pushes less: R=m(g−a). In free fall (a=g), it does not push at all. In orbit, the craft and everything in it fall around the Earth together, so the floor never pushes: the astronauts are weightless, although gravity is still pulling them.
Watch out. Weightless does not mean "no gravity". At the space station's height, g is still about 8.6m/s2.
A worked example
A 70kg astronaut is in the space station, 420km up.
. Polar satellites pass over the poles in low orbits and see the whole Earth as it turns beneath them. Astronauts in orbit feel weightless because they and their craft fall freely together, not because gravity is absent.
about 25 min1 simulation2 practice questions1 Python program4 questions to check yourself
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Answer, and give a reason. At 550 km up, where a space telescope orbits, is g zero?