2.6Chapter 2: Forces

Round in circles — Alhussan Physics - Stage 7 Revision Notes & Topical Questions

5 min read 7 parts Lesson 6 of 7 1 quiz

📚 Alhussan Physics - Stage 7 Hub / Topical Revision

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Lesson summary

2.6 Round in circles — Gravity and Planetary Orbits — Alhussan International Academy — Physics (Stages 7–9) notes: Explain what a centripetal force is, why some objects move in circles and how gravity keeps planets, moons and satellites in orbit.Effects of a force — a force can c…

Along the motion
At right angles
Trolley example
Hammer throw
Car on a bend
Cyclist on a bend

Part 1 of 5A force can change direction

Effects of a force

Effects of a force a force can change the shape, speed or direction of motion of an object.

Along the motion

Along the motion when a force acts in the direction of motion, the object speeds up or slows down.

At right angles

At right angles when a force acts at right angles to the motion, the object keeps turning instead of moving straight.

Trolley example

Trolley example a trolley pushed from behind moves in a straight line,
but a steady sideways push applied all the time bends its path and it starts to move in a circle.

Part 2 of 5Centripetal force

Centripetal force

Centripetal force a force that keeps an object moving in a circle.

It is always directed towards the centre of the circle.
It is always at right angles to the direction of motion of the object.

Hammer throw

Hammer throw the thrower swings a big metal ball on a wire around her head.
The tension in the wire provides the centripetal force and changes the direction of motion.
If she lets go, the ball moves along a tangent to the circle.

Car on a bend

Car on a bend friction between the tyres and the road provides the centripetal force that makes a car go around a corner.

Cyclist on a bend

Cyclist on a bend the force comes from friction between the tyres and the road, with the cyclist leaning in.

Force switched off

Force switched off the object moves in a straight line along the tangent in the direction it was moving at that instant, not towards the centre or outwards.

Part 3 of 5Gravity as the centripetal force in space

Orbits

Orbits the Earth and other planets orbit the Sun, and the Moon orbits the Earth as a natural satellite.

Artificial satellites

Artificial satellites they are put into orbit around the Earth for communication and for studying the surface.

Gravity

Gravity a force of attraction between all objects with mass, and it supplies the centripetal force in each case.

The Sun's gravity keeps the planets and asteroids in orbit around the Sun.
The Earth's gravity keeps the Moon and artificial satellites in orbit around the Earth.
A planet's gravity keeps its moons in orbit, such as the moons of Jupiter.

Balance

Balance the planet is continuously pulled towards the Sun,
so its path curves instead of going straight, and the orbit is a balance between sideways motion and the inward pull.

Orbiting object	Object it orbits	Force providing the centripetal force
Planet or asteroid	Sun	Gravity
Moon, artificial satellite	Earth	Gravity
Moons of Jupiter	Jupiter	Gravity
Ball on a wire	Centre of the circle	Tension
Cyclist or car on a bend	Centre of the bend	Friction

Part 4 of 5Why satellites do not fall to the ground

1Falling and moving sideways

Falling and moving sideways a satellite is always falling towards the Earth because gravity pulls it inwards,
- but it is also moving sideways very fast.

2Result

Result as it falls, the surface of the Earth curves away beneath it, giving a circular or nearly circular orbit.

3Changing speed

Changing speed if a satellite slowed down it would spiral towards the Earth, and if it sped up its orbit would become larger.

4Orbit times

Orbit times the Moon takes about 27 days to orbit the Earth once.
A satellite in geostationary orbit takes 24 hours, the same time as the Earth takes to spin, so it stays above the same point on the equator.
Planets further from the Sun are pulled less strongly and take longer to complete an orbit.

Part 5 of 5Satellites in society

1Communication and navigation

Communication and navigation satellite systems such as GPS have changed people's lives, usually for the better.

2Space junk

Space junk there are lots of old satellites and other junk now orbiting the Earth, which pose a risk to other satellites and may fall to the Earth.

Key terms

Term	Meaning
Centripetal force	a force directed towards the centre of a circle that keeps an object moving in a circle.
Orbit	the curved path of one object around another.
Natural satellite	a moon, a natural object that orbits a planet.
Gravity	a force of attraction that acts between all objects with mass.
Tangent	a straight line touching a circle at one point, in the direction the object would move if the force stopped.
Artificial satellite	a human-made object placed in orbit.

Avoid these mistakes

Thinking an object moving in a circle has a force pushing it forwards along the circle — the force points towards the centre at right angles to the motion.

Thinking a switched-off centripetal force sends the object towards the centre or outwards — it moves in a straight line along the tangent.

Believing planets stay in orbit because there is no force on them — gravity acts all the time and keeps them orbiting.

Calling the Moon a planet — it is a natural satellite because it orbits a planet.

Last updated: 13 October 2026