The Greek thinker Aristotle (384 B.C. – 322 B.C.), held the view that if a body is moving, some external force is required to keep it moving. Thus, the Aristotelian law of motion is : An external force is required to keep a body in uniform motion. It is a natural view as external forces seem to be needed to keep bodies in motion. Left to themselves, all bodies eventually come to rest.
But there is flaw in Aristotle’s argument. A ball rolling on the floor stops because an external force of friction between the ball and the floor and also resistance of air oppose the motion of the ball. If there were no opposing forces, the rolling ball would never stop.
In fact, the opposing forces such as friction (in case of solids) and viscous forces in case of fluids) are always present in the natural world. That is why forces by external agencies are necessary to overcome the opposing forces to keep bodies in uniform motion. If we were to imagine a world without any frictional forces, no force would be required to keep a body moving uniformly along a straight line. This is the true law of nature for forces and motion.
Frequency Asked Questions
No, if there is no force of friction. External force is only required to overcome the frictional and viscous forces.