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Unit 3: Gravity and Electromagnetism

Quick questions on Satellite and orbital motion: WACE Year 12 Physics Unit 3

2short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.

What is gravity supplies the centripetal force?
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For a satellite of mass mm orbiting a body of mass MM at radius rr, the gravitational force is the centripetal force:
What are geostationary satellites?
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A geostationary satellite stays above a fixed point on the equator, so its period equals one sidereal day (about 2424 hours) and it orbits west to east. Putting T=86400 sT=86400\ \text{s} and M=MEM=M_E into Kepler's law gives an orbital radius of about 4.2×107 m4.2\times10^7\ \text{m}, roughly 36000 km36000\ \text{km} above the surface. These orbits are used for communications and weather satellites because the antenna can point at one spot in the sky.

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