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Newton's laws of motion are very important in the classical physics. A large number of principles and results may be derived from Newton's laws. The first two laws relate to type of motion of a system that results from a given set of forces. These laws also help to define certain terms like mass , acceleration and force . Thus, these laws are kind of like definitions.


  • Define the first three laws with brief explanation
  • Understand how the change of frame affects the application of these laws
  • Create a systemic algorithm for making equations while problem solving
  • Understand the concept of Pseudo forces
  • Define Inertia

First Law of Motion

If the vector sum of all the forces acting on a particle is zero then and only then the particle remains unaccelerated (i.e. either at rest or in uniform motion).

This concept is meaningful, however, only when a frame of reference is specified. When we look at the acceleration of the particle of different frames we find it to be different.

Figure- LM01

Consider a situation illustrated following(Hcv64). An elevator cabin falls down after the cable breaks. The cabin and all the bodies fixed in the cabin are accelerated with respect to the earth and the acceleration is about 9.8 m/s2. in the downward direction.

Consider the lamp in the cabin. THe forces acting on the lamp are the gravitational force W by the eath and the electromagnetic force T (tension) by the rope. Th direction of W is downward and the direction of T is upward. The sum is (W-T) downward. Measure the acceleration of the lamp from the frame of reference of the cabin. The lamp is at rest. The acceleration of the lamp is zero. The person A measured this acceleration and uses Newton's first law to conclude that the sum of the forces acting on the particle is zero i.e.,


Instead if we measure the acceleration from the ground, the lamp has an acceleration of 9.8 m/s2. Thus, W - T ≠ 0 or W ≠ T. Will surely complete this article soon. You can also help complete the topic for second law.