Read each statement below carefully and tick the correct statements only a) A particle in one-dimensional motion with constant speed must have zero acceleration. b) A particle in one-dimensional motion with a positive value of acceleration must be speeding up. c) A particle in one-dimensional motion with zero speed may have non-zero velocity. d) A particle in one-dimensional motion moving with constant velocity will have average velocity equal to instantaneous velocity.
Answers
(a) True, when a body is thrown vertically upwards in the space, then at the highest point, the body had zero speed but has downward acceleration equal to the acceleration due to gravity.
(b) 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.
(c)True, when a particle is moving along a straight line with a constant speed, its velocity remains constant with time. Therefore acceleration is zero.
(d) The statement depends upon the choice of the instant of time taken as origin, when a body is moving along a straight line with positive acceleration. The velocity of the body at an instant of time t is
v=u+at
The given statement is not correct if a is positive and u is negative, at the instant of time taken as origin. Then for all the times before the time for which v vanishes, there id slowing down of the particle i.e. the speed of the particle keeps on decreasing with time. It happens when the body is projected vertically upwards.
However the given statement is true if u is positive and a is positive, at the instant of time taken as origin. It is so when the body is falling vertically downwards