GCSE course · Lesson 4 of 12

Forces and orbits

Asteroids, planets and meteoroids all move under gravity. This lesson applies Newton's laws and circular motion to orbits, and explains how a fragment's orbit can change.

In this lesson you will

Explain why an object in a circular orbit is accelerating.
Calculate orbital speed from radius and period.
Explain how small forces change orbits over long times.

Links to: GCSE physics, forces: Newton's laws, velocity as a vector and resultant force; separate physics, space physics: circular orbits and the effect of speed on orbital radius.

Orbits need a force

Newton's first law says an object keeps moving at a constant velocity unless a resultant force acts. Velocity includes direction, so an object moving in a circle at a steady speed is constantly changing velocity, which means it is accelerating towards the centre.

For an orbit, the force providing that acceleration is gravity. The Sun's gravity pulls each planet and asteroid towards it; their sideways motion carries them around rather than into the Sun. For a stable circular orbit, a body closer to the Sun must move faster, because the pull of gravity is stronger there.

Elliptical orbits

Most real orbits are ellipses, stretched circles. Many meteorites come from asteroids with elliptical orbits that dip in towards Earth's orbit at their closest point to the Sun and stretch out to the asteroid belt at their furthest. When scientists reconstruct a fireball's path from camera images, they can calculate the meteoroid's orbit and usually trace it back to the main asteroid belt.

Changing an orbit

Collisions and gravity change orbits. Two other effects matter for small bodies. Repeated gravitational tugs from Jupiter at certain resonant distances stretch an orbit. A small spinning body also absorbs sunlight and re-emits heat unevenly, which gives it a tiny push that slowly drifts its orbit over millions of years. Together these steer fragments from the asteroid belt into Earth-crossing orbits.

Worked example

How fast is Earth moving?

Earth orbits the Sun at a radius of 1.5 × 10¹¹ m and takes one year, 3.2 × 10⁷ s, to go round once. Assuming a circular orbit, calculate its orbital speed.

Distance travelled in one orbit = circumference = 2πr = 2 × 3.14 × 1.5 × 10¹¹ ≈ 9.4 × 10¹¹ m.
speed = distance ÷ time = (9.4 × 10¹¹) ÷ (3.2 × 10⁷).
9.4 ÷ 3.2 ≈ 2.9 and 10¹¹ ÷ 10⁷ = 10⁴, so speed ≈ 2.9 × 10⁴ m/s.

Answer: About 2.9 × 10⁴ m/s, or 29 km/s. A meteoroid meeting Earth head on adds this to its own speed, which is why some meteors reach about 70 km/s.

Activity

Swing a soft ball or bung on a string in a horizontal circle (with plenty of space and eye protection). Describe what you feel through the string, and predict what happens if you let go. Link your answers to gravity providing the force for an orbit.

Quiz

5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.

Question 1An asteroid moves in a circular orbit at constant speed. Which statement is true?
Question 2What provides the force that keeps a planet in orbit?
Question 3For stable circular orbits, how does speed change with distance from the Sun?
Question 4A circular orbit has radius 2.0 × 10⁸ m and period 1.0 × 10⁵ s. What is the orbital speed? (use π = 3.14)
Question 5How do scientists find where a fireball's meteoroid came from?