IIT JEE , AIEEE Forum - Physics , Mechanics

madman

there is a spring attached to a wall on one end and block on the other

a force F acts on the block

now suppose i want to find the maximum elongation

i can write

F=k x(max)

because at maximum displacement force applied = spring force

so x(max) = F/k

or i could conserve energy as

1/2 kx^2 = Fx

x = 2F/k

which is the correct one and what is the flaw?

dinesh_ddt

second one is correct
coz in first x will give the pos where net force is zero but due to inertia it will move further

Conjurer

Its always energy in such cases because it neglects every other disturbance like velocity and other stuff.

madman

i understand that energy is the best way and all that
but i want you to find the flaw.
i actually have something in mind but i am not too sure
because i havent been able to verify it so i want you guys to take a shot

madman

someone?
is no nerd online?

chemicaltester

see madman
use of conservation of energy is best

i cannot find how did u apply straight way force f as constant
as f is changing all the time although periodically
when u will use integration and find equation then
plkace the displacement as max in equation u will find there will be different asnwer
probably the same as u r getting in secound equation

i may be wroeng
please rate me if i m right

Ksudha_iit

hey ....i want to know some answers....a few queries...is the force consant...the one which is acting..? n wt about the mass of the block??? why didnt u ppl consider gravitational potential energy?? pls answer to these n i shall give u the explanation. and initially the spring would have been elongated a bit due to the force acting n due to gravity....so in the energy theorem u should modify ....wt about all these ...why none of u considered ???

varun.tinkle

IF THE FORCE ACTING ON IT IS CONSTANT THEN F=KX
HOW CAN U USE THE PTINCIPLE OF CONSERVATION OF ENERGY WHEN THERE IS AN EXTERNAL NON-CONSERVATIVE FORCE IS ACTING ON THE SYSTEM

PLS RATE ME IF U FIND ME USEFUL.
CHEERs!!!!!!!!!!

karthik2007

@madman - kx=F means that the net force is zero, but the spring can still move with constant velocity which means that there still will be elongation. If you conserve energy, all the work done by F is used in expanding the spring, hence it is more correct.

varun.tinkle

see karthik how does the equation f--kx tells that the force applied=0
the only condition in this equaton is that there should be no constant force acting on the system

madman

@every one who told force is not constant and all that
when did i say it was constant
i only considered the situation at max elongation when net force acting on THE BLOCK is zero
i think all of you thought the force is applied and then the block is left to move
i am telling you that a constant force is applied on the block till it comes to rest and at maximum elongation the force on it is zero

@karthik
how will the spring have any velocity
it should be temporarily stationary at max. elongation

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