Question 3.11 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 that instant
(b) with zero speed may have non-zero velocity,
(c) with constant speed must have zero acceleration,
(d) with positive value of acceleration mustbe speeding up.
Class XI Physics Motion In A Straight Line Page 57
Answers
(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
Answer:
(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