Orbits

Overview

This lesson applies gravitational force to circular orbital motion. You will treat gravity as the centripetal force for satellites and planets, then use this model to explain the special conditions needed for a geostationary orbit.

What You Need to Know

• In a circular orbit, gravitational force provides the centripetal force.
• The orbit radius is measured from the centre of the attracting mass, not from its surface.
• Orbital speed, angular speed, radius, and period are linked through circular motion equations.
• A geostationary satellite remains above the same point on Earth’s surface.
• A geostationary orbit has a period of 24 hours, is directly above the equator, and moves from west to east.
• The orbital radius is fixed by the central mass and the required orbital period.

How to Work Through It

1. Start by recalling centripetal acceleration and circular motion equations.
2. Set gravitational force equal to centripetal force for a circular orbit.
3. Practise using radius, period, angular speed, and orbital speed consistently.
4. Explain each condition for a geostationary satellite in physical terms.

Check Your Understanding

• Why is gravity not balanced by a separate outward force in a circular orbit?
• Why must orbital radius be measured from Earth’s centre?
• What period must a geostationary satellite have?
• Why must a geostationary orbit be above the equator?

Common Mistakes

• Adding an extra “centrifugal force” instead of using gravity as the centripetal force.
• Using altitude as radius without adding Earth’s radius.
• Forgetting to convert orbital period into seconds.
• Saying a satellite is geostationary just because it has a circular orbit.

Next Steps

• Practise one full orbit calculation with clear units.
• Bring field strength, gravitational force, potential, and orbits together in revision.