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Place a bag of sugar on the table, and gently push it. The sugar starts to move, BUT crucially when you stop
pushing it, it generally stops moving. Now if F=ma is true, this shouldn’t happen. We should give it a push,
and it will start to accelerate, say from 0 m/s to 1 m/s to 2 m/s. If we push it harder, we might get it to 3 m/s,
but it will still stop in short order.
So this looks much more like F=mv - (v is velocity - or speed in everyday language) the harder you push something the faster it goes, and it you keep pushing with the same force, it will keep going at the same speed.
That isn’t what Newton said though, according to him a quick push will set something in motion, and it will
move along happily for ever after. In fact this is his first law, which roughly translates (partially) as something
at rest will stay at rest unless you give it a push.
Our everyday experience is clouded by a hidden force, the force of friction. This is a force that resists motion.
It can be friction between a bag of sugar and a table, a wheel and the road, or an aircraft and the air. All
these act to resist the motion. In a perfect vacuum, say out in space, this is all so much easier to see. Give
an astronaut a push, and he will keep moving, which is both wonderfully liberating (I imagine) and a pain
in the neck after a while.
So anyway, it took insight to see that real equation is F=ma and not F=mv, and to then do experiments where
friction is factored out to prove it.
ICY SCIENCE | QTR 1 2014