Read each statement below carefully and state with reasons and examples if it is true or false.
A particle in 1 -D motion
(i) with zero speed at an instant may have non-zero acceleration at that instant.
(ii) with zero speed may have non-zero velocity.
(iii) with constant speed must have zero acceleration.
(iv) with positive value of acceleration must be , speeding up.
(i) True, when a body is thrown vertically upwards in the space, then at the highest point, the body has zero speed but has downward acceleration equal to the acceleration due to gravity.
(ii) False, because velocity is the speed of body in a given direction. When speed is zero, the magnitude of velocity of body is zero, hence velocity is zero
(iii) True, when a particle is moving along a straight line with a constant speed, its velocity remains constant with time. Therefore, acceleration (i.e., change in velocity/time) is zero.
(iv) False, if the initial velocity of a body is negative, then even in the case of positive acceleration, the body speeds down. A body speeds up when the acceleration acts in the direction of motion.