Physics · Laws Of Motion · NEET
The ball stops because of friction between the ball and the ground, plus air resistance. These are real external forces that oppose the motion. If you removed friction and air completely (a perfectly smooth surface in a vacuum), the ball would never stop. Galileo showed this using a double inclined plane: a ball rolling down one side climbs the other side to nearly the same height. Make the second side less and less steep, and the ball rolls farther and farther to reach that height. If the second side were flat and smooth, the ball would roll forever. So stopping is caused by friction, not by any need for force to keep going.
Aristotle's fallacy is the belief that an external force is always required to keep a body in uniform motion. He saw that a cart stops when you stop pushing, so he concluded that force is the cause of motion. His mistake was that he did not notice friction. In everyday life you must keep pushing only to balance the friction force. The push and the friction cancel, giving zero net force, and the body moves at constant speed. Force is not maintaining the motion; it is only cancelling friction.
It can have forces on it, but the net (total) force must be zero. On a real floor, your push balances friction, so individual forces exist but they sum to zero. In an ideal frictionless case, there is no force at all and the body still moves at constant velocity. The key idea for NEET: uniform velocity (constant speed in a straight line) always means zero net force, whether by cancellation or by having no force at all. This is exactly Newton's first law, which grew directly out of Galileo's law of inertia.
Galileo used idealisation, or extrapolation. He studied real inclined planes where he could reduce friction more and more (smoother surfaces). He observed that as friction got smaller, the ball travelled farther before stopping. He then imagined the limiting case of zero friction and concluded the ball would never stop. He could not build a frictionless surface, but by seeing the trend he reasoned to the ideal result. This method of removing a disturbing factor in the mind is a foundation of modern physics.
He was right that in daily life you must keep applying a force to keep an object moving, and wrong about why. The reason is friction, which he did not identify. So force is needed in practice, but only to counter friction, not to sustain the motion itself. Galileo separated these two ideas: the true law of nature (no force needed for uniform motion) and the practical situation (force needed to beat friction). NEET questions test whether you can tell these two apart.
A particle moving with velocity V is acted upon by three forces represented by the sides of a triangle PQR (taken in order). The velocity of the particle will:
A person standing on the floor of an elevator drops a coin. The coin reaches the floor in time t1 if the elevator is at rest, and in time t2 if the elevator is moving uniformly. Then:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the wrong idea that a body needs a constant external force to stay in uniform motion, when in truth force is only needed to balance friction.
He concluded that a body moving on a frictionless horizontal surface would keep moving at constant velocity forever, needing no force. This is the law of inertia.
Because friction and air resistance always oppose motion. You apply force only to cancel these, giving zero net force and steady motion.
Galileo's law of inertia is exactly what Newton stated as his first law of motion. Newton built his mechanics on Galileo's idea.
NEET often asks whether uniform velocity needs a force. The correct answer is always zero net force, and this concept underlies inertia and Newton's first law questions.
