HAVE you ever let the words “centrifugal force” escape from your lips? Shame on you: you might as well have called it the “hocus-pocus force”. You are in good company, though. Scientists, engineers and, we confess, even 91av, sometimes let the c-word slip.
Why can’t we help ourselves? It’s all down to our subjective experience getting on top of our scientific judgement. Drive round a curve too fast and you feel as if you’re being flung outwards. Turn right sharply, and your sunglasses slide off to the left along the dashboard. And if you enjoy fairground rides you will know that on the “sticky wall” you end up pinned against the inside of a vertical spinning drum as the floor drops away.
So, intuition aside, what’s really going on? It’s all down to Isaac Newton’s laws of motion. Stationary objects, Newton pointed out, stay put, and moving objects travel forever with the same velocity unless some force acts on them.
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As you round a bend, you may feel you are being flung outwards but in reality you are just trying to go straight on. Indeed, if you were pushed out of the car, gangster-movie style, while Newton hovered overhead in a police helicopter, he would see you continue in a straight line until you hit the ground.
What we should be talking about here is centripetal rather than centrifugal force. This name comes from the Latin words meaning “centre” and “seeking”. The centripetal force is what makes objects move in a circle. Our notional car, planes looping-the-loop, even planets moving around the Sun – they would all simply fly of at a tangent were it not for the force’s inward pull.
What does the force actually do? When a car is travelling round a bend at constant speed, its velocity is changing all the time, because velocity takes into account the direction of travel as well as the speed. Changing velocity equates to acceleration, and the force producing this acceleration is directed towards the centre of the circle that captures the curve.
Every schoolchild is taught all this at some time. But why do we then forget? Perhaps because everyday experience is just too compelling in this and other places where Newton’s laws still seem to surprise us.
Why else would so many science fiction movies show craft in deep space pushing themselves along, rockets firing, as though the craft would stop the moment they were switched off? And why would David Scott, commander of the Apollo 15 mission, have had to drop a feather and a hammer on the Moon in 1971, four centuries after Galileo? To the surprise of some, the hammer and the feather hit the lunar surface at the same time, just as the laws of physics predict.
What should you say if you are ever caught out in these errors? There’s always one engineer’s response: “I just use centrifugal as a shorthand for centripetal.”
