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Mechanics

Cool goIITian

 Joined: 5 Aug 2007 Post: 33
1 Jan 2008 20:34:34 IST
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Engineering Entrance , Medical Entrance , AIPMT , JEE Main , AIIMS , JEE Advanced , Physics , Mechanics

in hc verma
a boy climbs a ladder  (its vertical)

what is work done by the ladder on the boy ??

displacement is upward   force(normal rn) is also upward   ,theta is 0
(i think)

Blazing goIITian

Joined: 1 May 2007
Posts: 2830
1 Jan 2008 20:35:28 IST
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zero? The only force exerted by ladder is Normal reaction which is perpendicular to its displacement

Blazing goIITian

Joined: 9 Apr 2007
Posts: 451
1 Jan 2008 21:29:11 IST
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don`t you think that the ladder does work to overcome the potential energy of the boy....

Blazing goIITian

Joined: 16 Dec 2007
Posts: 432
2 Jan 2008 16:03:39 IST
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@karthik - the normal reaction is obviously upward . it is the force exerted by the ladder that helps the boy climb upward. take it this way - the boy applies a downward force to climb up, not a force that is perpendicular to the ladder - and hence the normal reaction of the ladder cannot be perpendicular to displacement. also, it is clear no other force can be responsible for this displacement.

@tarun - you are right yaar

work done by ladder = [0 ][s ] Fds

where F is the instantaneous force applied by the boy - because the instantaneous normal reaction is equal to this force.

Blazing goIITian

Joined: 9 Apr 2007
Posts: 451
2 Jan 2008 16:09:43 IST
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thats right

Blazing goIITian

Joined: 1 May 2007
Posts: 2830
2 Jan 2008 17:18:56 IST
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yes, tka is right... I didn't notice that vertical part properly

Cool goIITian

Joined: 5 Aug 2007
Posts: 33
4 Jan 2008 18:22:50 IST
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hello

i also think the same
but in the answers  the ans of the probm is 0  i donno y??
can h c v  be wrong!!!!
i dont think so
so may be we r wrong??

Blazing goIITian

Joined: 27 Dec 2007
Posts: 822
4 Jan 2008 18:33:06 IST
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hey the ansr is definitely 0 bcoz work done by ladder on the boy is zero......

Blazing goIITian

Joined: 16 Dec 2007
Posts: 432
6 Jan 2008 18:24:59 IST
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i beg to differ. if the work done by the boy on the ladder is finite, then so is the work done on the boy by the ladder. if you say it depends on the frame, well only then is there a small chance the answer is 0. i stick with what i said.

Blazing goIITian

Joined: 27 Dec 2007
Posts: 689
6 Jan 2008 19:52:56 IST
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here's what i think...................when the boy climbs the ladder, he exerts a downward force on the ladder........the ladder exerts an equal and opposite force on the boy...........now, when he climbs up, there are two forces performing work......gravity(Wg) and the ladder.(Wl)......

apply the work energy theorem
between the ground and the top of the ladder..........

we have

Wg+Wl=0(as initial KE=final KE=0)

now calculate work done by gravity, which is negative..........
mass=20
height=8
g=10
theta=180 degree
so
Wg=20.8.10.cos(180)
= -1600J

so
Wl=-(Wg)
= +1600 J

i don't think the answer is zero.........but then again, how can hcv sir be wrong>?

New kid on the Block

Joined: 2 Jan 2008
Posts: 5
6 Jan 2008 19:58:07 IST
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well friends,
WORK DONE= FORCE X DISPLACEMENT

In this case the displacement of the ladder is 0 and hence the wok done by it is zero.

Blazing goIITian

Joined: 27 Nov 2007
Posts: 470
6 Jan 2008 20:00:00 IST
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according to the book,the work is done by the internal forces of boy

Blazing goIITian

Joined: 27 Dec 2007
Posts: 689
6 Jan 2008 20:09:44 IST
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@urvashishami

i beg to differ.............the work done would have been zero if  the boy was not displaced..........work is   zero if the displacement of the body on which the force was applied were zero......isn't it?

Blazing goIITian

Joined: 16 Dec 2007
Posts: 432
7 Jan 2008 19:36:53 IST
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@budku:
work is done by internal forces of the boy? i am completely against that statement!!!

can the internal forces of the boy cause the boy himself to be displaced? i dont think so!!! i still stick with my answer.

@urvashi:
obviously , just because displacement of ladder is zero, work done is zero? no. thats only wrt ground frame. imagine this situation - i push a heavy box and it starts moving. i dont move from my place. does this mean the work done by me is zero??? its not.

Blazing goIITian

Joined: 16 Dec 2007
Posts: 432
7 Jan 2008 19:44:27 IST
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OKAY i have a doubt -

with respect to what frame is the question asked in the textbook?

and what page is this question in?

Cool goIITian

Joined: 5 Aug 2007
Posts: 33
12 Jan 2008 20:34:52 IST
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page:134

quest no 35
as there is disp of the boy ,the force is applied
work =force * disp??

so work is done by ladder on the boy to make it displace

we can take any frame of reff   (work is scalar)

Blazing goIITian

Joined: 16 Dec 2007
Posts: 432
13 Jan 2008 21:44:51 IST
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The only thing I can think of so as to suggest work done by ladder is 0, is that it is because of friction that he is able to climb up - that he grips it tightly. this says - work done by ladder is 0, work done by friction is what moves him up.

Blazing goIITian

Joined: 2 Jan 2008
Posts: 504
13 Jan 2008 22:58:08 IST
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hence a work is performed by him which is in direction of force applied
the fact that work =force *displacement
since the boy moves i.e displacement.e. work done
now we say that work done by LADDER ON BOY IS 0
this is due to 0 displacement
but how it can be??
initially it is said that displacement has taken place n now i m saying that there is no displcacement

the real fact is the displacement of the boy is account of his own energy
the ladder does not contribute in any sense the displacement of boy

hence 0

New kid on the Block

Joined: 14 Jan 2008
Posts: 2
14 Jan 2008 22:47:33 IST
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The work done by the boy should be zero. The gain in potential energy is the energy spend by the boy in climbing the ladder.

New kid on the Block

Joined: 14 Jan 2008
Posts: 2
14 Jan 2008 22:48:08 IST
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The work done by the boy should be zero. The gain in potential energy is the energy spend by the boy in climbing the ladder

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