We will discuss

Everybody continues to be in its state of rest or of uniform motion until and unless and until it is compelled by an external force to change its state of rest or of uniform motion. Inertia The property by virtue of which a body opposes any change in its state of rest or of uniform motion is known as inertia. The greater the mass of the body greater is the inertia. That is mass is the measure of the inertia of the body. Numerical Application If, F = 0 ; u = constant Physical Application 1. When a moving bus suddenly stops, the passenger’s head gets jerked in the forward direction. 2. When a stationery bus suddenly starts moving passenger’s head gets jerked in the backward direction. 3. On hitting a used mattress with a stick, dust particles come out of it. 4. In order to catch a moving bus safely, we must run forward in the direction of motion of the bus. 5. Whenever it is required to jump off a moving bus, we must always run for a short distance after jumping on the road to prevent us from falling in the forward direction. Key Concept In the absence of externally applied force-velocity of the body remains unchanged. Newton’s 2nd law Rate of change of momentum is directly proportional to the applied force and this change always takes place in the direction of the applied force.