OCR A asks you to explain, for circular orbits, how the force of gravity changes an object's velocity but not its speed, and how the radius of a stable orbit must change if its speed changes. This is qualitative only: there is no calculation.
- An object moving at constant speed in a circle is still accelerating, because its direction, and so its velocity, keeps changing.
- In a circular orbit the force of gravity pulls towards the centre, at right angles to the velocity. It changes the direction of the motion but not the speed.
- For a stable orbit, if the speed changes then the radius must change too: a faster orbital speed goes with a smaller radius.
Quick Check: A satellite orbits the Earth in a circle at a constant speed. Explain why its velocity is changing even though its speed is not.
Velocity has a direction as well as a size. The force of gravity pulls the satellite towards the centre of the orbit, so the direction of its motion keeps changing. The speed stays the same, but the velocity changes, so the satellite is accelerating.
This key facts covers OCR A (J249): Circular Orbits within Newton's Laws of Motion for GCSE Physics. Revise Newton's Laws of Motion in Forces for GCSE Physics with 18 exam-style questions and 15 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 14 of 17 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Newton's Laws of Motion
According to Newton's First Law, what happens to an object when there is no resultant force acting on it?
A spaceship is travelling through deep space far from any planets. The engines are switched off. Explain what will happen to the motion of the spaceship and why.