Newton's First Law of Motion Everybody continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise. Hence, Statement 1 is correct. The state of rest or uniform linear motion both implies zero acceleration. The first law of motion can, therefore, be simply expressed as: If the net external force on a body is zero, its acceleration is zero. Acceleration can be non-zero only if there is a net external force on the body . The first law refers to the simple case when the net external force on a body is zero. Newton's Second Law of Motion The second law of motion refers to the general situation when there is a net external force acting on the body. The momentum of a body is defined to be the product of its mass m and velocity v and is denoted by p: [p = mv] Momentum is clearly a vector quantity. The same force for the same time causes the same change in momentum for different bodies. The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts. Hence, Statement 3 is incorrect. Newton's Third Law of Motion To every action, there is always an equal and opposite reaction. Hence, Statement 2 is incorrect. There is no cause-effect relation implied in the third law. Consider a pair of bodies A and B. According to the third law, F AB = – F BA [(force on A by B) = – (force on B by A)]
