Laws of MotionAristotle's Fallacy
The question sounds simple, yet it took about two thousand years to answer. The Greek thinker
It is easy to see why. Pull a toy car along by a string and it moves; let go and it soon stops. Sina's toy car has a mass of 1 kg and is already rolling at 2 m s⁻¹. Choose a floor and how hard Sina pulls the string, then run the car for 4 s and watch its velocity–time graph.
The velocity–time graph came out flat only when the forces on the car cancelled. On the carpet the amber friction arrow pulled backward all the time, and the car kept its speed only when Sina's pull was exactly as big. On the ideal floor there was no friction, and no pull was needed at all. While the toy car rolled at a steady 2 m s⁻¹ on the carpet, the net force on it was
That is Aristotle's mistake. A toy car stops because the friction of the floor opposes its motion; the child's pull is needed only to cancel the friction. In steady motion the two cancel, and the net external force is zero. With no friction, no pull would be needed at all.
Note
Friction between solids, and viscous drag in liquids and gases, are everywhere in the world around us, which is why we must keep pushing things to keep them moving. Aristotle turned that everyday experience into a law of nature. To find the true law you have to imagine a world without friction. That is what Galileo did.