Q. A MAN IS ‘L’=9 m BEHIND THE DOOR OF A TRAIN WHEN IT STARTS MOVING WITH ACCELERATION a=2m/s2. THE MAN RUNS AT FULL SPEED. HOW FAR DOES HE HAVE TO RUN AND AFTER WHAT TIME HE GET INTO THE TRAIN? WHAT IS HIS FULL SPEED?

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Grade Select GradeMechanics

Q. A MAN IS ‘L’=9 m BEHIND THE DOOR OF A TRAIN WHEN IT STARTS MOVING WITH ACCELERATION a=2m/s². THE MAN RUNS AT FULL SPEED. HOW FAR DOES HE HAVE TO RUN AND AFTER WHAT TIME HE GET INTO THE TRAIN? WHAT IS HIS FULL SPEED?

Bharat Makkar

11 Years agoGrade Select Grade

4 Answers

shashi K Sharma

Approved Tutor Answer11 Years ago

Dear Student

There is a huge conceptual error with the author of the question who thinks that the train can be boarded only when the speed of the body say v equals the speed of train, but blv me theres no such contraint practically;

Anyhow let us honour the question then speed of body v= speed of train at the time of boarding say at time t

hence v = at or v=2t

also the man travels a distance vt in time t which should be equal to the gap of 9m plus the distance moved by train in time t(which is 1/2at^2)

i.e. vt = 9 + 1/2at^2

replacing v=2t and a=2 we get

2t^2 = 9 + t^2

or t=3

hence vt =

shashi K Sharma

11 Years ago

thus full speed v = at

or v = 6m/s

Bharat Makkar

11 Years ago

thanks sir

Richik

9 Years ago

Sir i have a doubt.sir,is it necessary that the time instant when the man will attain the velocity of the train will be the time instant where he covers the distance of the train +9m because that is what u have done.

V=2t will be valid only for a particular time instant and that need not be the time in which man will cover the distance s +9m.Please clarify