question_answer 11)

Read each statement below carefully and state with reasons and examples, if it is true or false. A particle in one-dimensional motion: (a) with zero speed at an instant may have non-zero acceleration at the instant, (b) with zero speed may have non-zero velocity, (c) with constant speed must have zero acceleration, (d) with positive value of acceleration must be speeding up.  

Answer:

(a) True. When a body begins to fall freely under gravity, its speed is zero but it has non-zero acceleration of . (b) False. Speed is the magnitude of velocity and the magnitude of non-zero velocity cannot be zero. (c) True. When a particle moves with a constant speed in the same direction, neither the magnitude nor the direction of velocity changes and so acceleration is zero. In case a particle rebounds instantly with the same speed, its acceleration will be infinite which is physically not possible. (d) False. If the initial velocity of a body is negative, then even in case of positive acceleration, the body speeds down. A body speeds up when the acceleration acts in the direction of motion.