Lesson focus: The Solar SystemThe Sun and everything that travels around it, including the planets and their moons. is held by gravityThe force that pulls objects towards each other, such as the Earth pulling you down. while orbital motion continually changes direction.
Order the planetA large object that travels around a star, like the Earth around the Sun. and explain orbitThe curved path one object takes around another, like the Earth around the Sun. using inward forceA push or a pull, measured in newtons (N). and tangential velocity.
Build on earlier learning
Balanced and unbalanced forcesApply inward resultant force to continuous circular/elliptical motion.
You will be successful when you can…
- Name Solar System components.
- Order eight planets.
- Draw inward gravity and tangent velocity.
- Explain continuous orbital curvature.
Words that carry the science
- Solar System
- Sun and objects gravitationally bound to it.
- orbit
- Curved path under gravitational attraction.
- tangent
- Straight direction touching a curve at one point.
- gravity
- Attractive interaction between masses.
- scale model
- Representation using a stated size or distance ratio.
Combine inward gravity with sideways motion
01The family
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune orbitThe curved path one object takes around another, like the Earth around the Sun. the Sun; dwarf planets, moons, asteroids and comets also belong.
02Why the path curves
gravityThe force that pulls objects towards each other, such as the Earth pulling you down. points inward toward the Sun and changes velocity direction. Tangential motion carries the planet onward.
Imagine swinging a ball on a string. The string pulls the ball inwards, and the ball keeps moving sideways, so it goes round in a circle. For a planet, the Sun’s gravityThe force that pulls objects towards each other, such as the Earth pulling you down. is the string: it pulls the planet towards the Sun, while the planet keeps moving sideways. Together these make the curved path called an orbitThe curved path one object takes around another, like the Earth around the Sun..
03Scale carefully
Planet sizes and orbital distances need different classroom scales. Diagrams are usually not to scale.
Orbits in the Solar System
All eight planets orbit the Sun. The further a planet is from the Sun, the longer its year.
Controls
Predict without gravity
Imagine the Sun’s gravity vanished instantaneously.
- Planet already has tangential velocity.
- Without force, velocity would not curve.
- It would continue approximately in a straight tangentA straight line that just touches a curve at one point. line.
- Therefore gravity is what continually bends the actual path.
How to make the reasoning visible: Orbit is falling around, not hovering in balance.
Solar System order
Use order and broad type, not memorised drawing sizes.
| Position | Planet | Broad type | Evidence note |
|---|---|---|---|
| 1 | Mercury | Rocky | Closest planet |
| 3 | Earth | Rocky | One Moon |
| 5 | Jupiter | Gas giant | Largest planet |
| 8 | Neptune | Ice giant | Most distant major planet |
Name planets between Earth and Jupiter.
Why is a single scale difficult?
Use the table. Name the largest planet and say what type it is.
Check your understanding
Answer from memory, then check against the suggested answer.
What is Solar System?
What is orbit?
What is tangent?
What is at the centre of the Solar System?
Which force keeps the planets in orbit?
Why does Earth not fall straight into the Sun?
Would zero resultant allow a circular orbit?
Why do planets further from the Sun take longer to complete one orbit?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A planet moves in an orbit (a curved path) around the Sun. Answer the parts to explain the role of gravity.
What does the Sun’s gravity do to the planet?
(1)Which way does gravity pull the planet?
(1)Which way is the planet already moving?
(1)What do the pull and the sideways movement make together?
(1)Total for this question: 4 marks
Command word: Think of the ball on a string in section 02.