Sheila Terry/SPL
1 A fourteenth century illustration showing the circular paths of heavenly bodies.
2 Centripetal force
The ancient Greek philosophers believed that heavenly bodies (Sun, Moon, stars and planets) all moved in circles (1) — a ‘perfect’ form of motion suitable for such objects. This idea persisted for centuries until Isaac Newton (1642–1727) showed that, in the absence of any net force, an object will remain at rest or travel with constant speed in a straight line (constant velocity).
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