IIT JEE , AIEEE Forum - Physics , Mechanics

LAMPARD

A rocket of mass m is fired upward.As soon as it is fired,it is observed that the rocket just lifts off the test stand but does not achieve sufficient thrust to move upward.The force exerted by the gases on the test stand during the failed firing process is best represented by which graph???

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are there some choices in this qsn by any chance????

abhishekray07

where are the graphs?

rtiit

can u give options?????

LAMPARD

I cant upload the pictures of the graph because when i try to upload an image,firefox gets hanged.
But i can describe them..
The axes are F along y-axis and t along x-axis.
a)At t=0,F is mg and as t increases,F is constant,i.e. straight line.
b)At t=0,F is 0 and then increases with x as a straight line.(y=mx type)
c)At t=0,F is mg and as t increases,F goes on decreasing.(not in a straight line,curved type)
d)At t=0,F is 0 and first,F increases upto a value of t and then decreases as t increases(not straight line but curved path)

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in D, does F increase till F = mg??
if it doesnt then the ans is C

PS: lampard.. pls check if my soln in that AC sum is rite..

LAMPARD

Nope,in d,it doesnt increase till mg.
And answer is not c.

computer001

ganesha1991

isn't it a

the force remains constant
in general if a rocket is fired and continues in motion
F=ma
a increases
but m decreases
so f is constant

CORRECT ME IF I'M WRONG PLZZZ

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if its not C, then it has to be D.. but if its D, then F MUST go up to mg coz its given just lifts off no??

ganesha1991

d???
how??
can sumbody explain

LAMPARD

Really surprising that no one thought about b....the answer is given as b...but i am not convinced about the explanation given...its given that,as the rocket lifts off,it can just support its own weight which decreases as the mass decreases.But how can we say that it will be of the type y=mx???

adityaraj

answer iS d

LAMPARD

Anyone???

abhishekray07

check your nugdebook

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