Unit 9
Circular motion and gravity.
From a ball on a string to the Moon: centripetal force, universal gravitation, orbits and angular momentum.
Before this unit
- Forces
- Trigonometry Mathematics
The unit, lesson by lesson
Lessons
5 lessons · 28 practice questions · about 2 hours
Acceleration towards the centre
A ball going round a circle at a steady speed is accelerating all the time. Which way, by how much, and how long does one turn take?
The force that holds it on the circle
Moving on a circle takes a net force towards the centre, of size m·v²/r. What supplies it, how strong is it, and why is there no force pushing outward?
Universal gravitation
Any two masses pull on each other, with the same force, which grows with the masses and weakens with the square of the distance. From the pull between two bodies in the Scene to your own weight.
Orbits: a fall that never ends
A satellite is a body that keeps falling and keeps missing the Earth. Gravity is its centripetal force, and from that come its speed, its period, Kepler's third law, and why astronauts float.
Angular momentum: a skater and a gyroscope
A spinning body keeps its angular momentum, L = Iω, as long as no torque acts on it: a skater who pulls in her arms and spins faster, the energy she adds, and a gyroscope that circles its stand instead of falling.