Why gravity keeps moons, planets and satellites in orbit, how the velocity can change while the speed stays the same, why closer orbits are faster, and calculating orbital speed with v = 2πr/T.
What you will learn
- Explain how gravity changes the velocity but not the speed in a circular orbit
- Explain why a stable orbit at a smaller radius needs a higher speed
- Calculate orbital speed, period or radius with v = 2πr/T, converting km, hours, days and years (beyond the specification)
Gravity bends the path
A moon, a planet or a satellite would travel in a straight line if nothing pulled it.
Gravity pulls it towards the body it orbits, at right angles to its motion.
- In a circular orbit the speed stays the same, but the direction changes all the time.
- Velocity includes direction, so the velocity changes: the object is accelerating towards the centre.
- The resultant force causing this is gravity, acting towards the centre of the orbit.
AQA and OCR test the ideas in this section and the next on Higher papers only. Edexcel tests them on both tiers.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Gravity bends the path
- Closer means faster
- Orbital speed (beyond the specification)
- Try it: Change the orbit and watch the year
- Try it: Your first orbital speed
10 practice questions, marked as you go; about 25 minutes in all.
Key terms
Direction
In this unit: Space physics
The solar system and the Milky Way, the life cycle of a star, orbits and orbital speed, red-shift and the expanding universe, and what is still unknown.