8 · IGCSE Physics
Orbital speed
Set the radius and period of a circular orbit and see the orbital speed v = 2πr/T, then compare it with the planets of the Solar System or with satellites around the Earth.
Orbital speed comparison
v = 2πr / T
r in m, T in s, v in m s⁻¹
(distance travelled in one orbit = 2πr)
r in m, T in s, v in m s⁻¹
(distance travelled in one orbit = 2πr)
r =
T =
2πr =
v =
Try this
- Load Earth and check that v ≈ 29.8 km s⁻¹; then load Neptune. How does orbital speed change as planets get further from the Sun?
- Keep r fixed and drag T to about double its value. What happens to v? Check with v = 2πr/T.
- Switch to Earth satellites and compare the ISS with a geostationary satellite: which is faster, and why must the geostationary period be about 24 hours?
Common error: putting r in km and T in days or hours straight into v = 2πr/T – convert to metres and seconds first to get v in m s⁻¹.