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Solution


Q.

Which of the following statements is not correct
1 The zero velocity of a body at any instant does not necessarily imply zero acceleration at that instant.  
2 The kinematic equations of motions are true only for motion in which the magnitude and the direction of acceleration are constants during the course of motion.  
3 The sign of acceleration tells us whether the particle's speed is increasing or decreasing.  
4 All of these  

Ans.

(3)

The sign of acceleration does not tell us whether the particle's speed is increasing or decreasing. The sign of acceleration depends on the choice of the positive direction of the axis. For example, if the vertically upward direction is chosen to be the positive direction of the axis, the acceleration due to gravity is negative. If a particle is falling under gravity, this acceleration though negative results in increase in speed.