Coupon Accepted Successfully!



In this unit, we shall investigate the cause of motion and it will be found that a body at rest may be set into motion, when some force is applied on the application of an external force on it. The motion of the objects can be understood by finding out the relation between the force applied on a body and the effect produced on the state of rest or state of motion of the body ,by the applied force.


In this chapter, we are going to learn about
  • Aristotle’s Fallacy
  • The law of inertia
  • Newton’s first law of motion
  • Interpretation of first law
  • Momentum
  • Newton’s second law of motion
  • Interpretation of second law
  • Impulse
  • Newton’s third law of motion
  • Momentum
  • Conservation of momentum
  • Equilibrium of particle
  • Common forces in mechanics
  • Friction
  • Rolling friction
  • Circular motion

Aristotle's Fallacy

The great Greek thinker, Aristotle (384 B.C. - 322 B.C.), suggested that if a body was moving, something external must be required to keep it in that state and prevent it from coming to a stop. For example, an arrow shot from a bow keeps flying since the air behind the arrow keeps pushing it. Most of the Aristotelian ideas on motion are now known to be wrong and need not concern us. For our purpose here, the Aristotelian law of motion may be phrased thus: An external force is required to keep a body in motion.

What is the flaw in Aristotle's Law? The answer is: a moving toy car comes to a stop, because the ever-present external force of friction on the car by the floor opposes its motion. To counter this force, the child has to apply an external force (by her hands) on the car in the direction of motion. When the car is in uniform motion, there is no net external force on it: the force by the child cancels the force (friction) by the floor. Therefore if there were no friction, there would be no need for the child to apply any force to keep the toy car in uniform motion.

The opposing frictional forces (between solids) or viscous forces (between solids and fluids, etc.) are always present in the natural world. This explains the practical experience that forces by external agencies are necessary to counter the frictional forces to keep bodies in uniform motion. Now we understand where Aristotle went wrong. It is in coding the practical experience in the form of a basic law.

To get at the true law of nature for forces and motion, one had to imagine an idealized world, in which uniform motion took place with no opposing frictional forces. This is what Galileo did.

Test Your Skills Now!
Take a Quiz now
Reviewer Name