Orbital Motion & Satellites — A/L Physics

What must I remember about Orbital Motion & Satellites?

• Orbital Velocity (v_o): Speed required to maintain stable circular orbit of radius r = R + h: Gravitational force provides centripetal force: (G M m) / r² = (m v_o²) / r => v_o = √(G M / r) = √(G M / (R + h)).
• Orbital Period (T) & Kepler's Third Law: Period T = (2π r) / v_o = 2π √(r³ / (G M)); T² = (4π² / (G M)) r³ (Kepler's Law of Harmonies: T² ∝ r³).
• Geostationary Satellites: Satellite that appears permanently fixed above a specific point on the equator: Period T = 24 hours (86,400 s); orbits in the equatorial plane from west to east; orbital altitude h ≈ 35,800 km (orbital radius r ≈ 42,200 km).
• Polar vs Equatorial Satellites: Polar orbits pass over Earth's poles at lower altitudes (200-1000 km) for global reconnaissance/mapping; Geostationary orbits are reserved for telecommunications and weather monitoring.

Test yourself (1 minute)

1. Apparent Weightlessness in Orbit

   Astronauts in an orbiting spacecraft feel weightless not because gravity is zero (g at 400 km is ~90% of surface value), but because the astronaut and spacecraft are in continuous free-fall together toward Earth.

2. Close-orbit velocity around Earth (h ≈ 0)

   v_orbit = √(g0 R_E) ≈ √(9.81 × 6.37 × 10⁶) ≈ 7.9 km/s.

3. Orbital decay

   Residual atmospheric friction at low altitudes dissipates satellite mechanical energy, causing orbital radius to shrink and velocity to paradoxically increase before burning up.

Practise Orbital Motion & Satellites questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 5: Gravitational Fields

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Unit 5.4 · Year 1 · Term 3