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22 May 2007 17:28:47 IST
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doubt in phy jee experts pls help
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Sir, i have a doubt in the reason assertion question asked in jee 2007
the question is
STATEMENT 1 a cloth covers a table. Some dishes are kept on it.The cloth can be pulled without dislodging the dishes from table
STATEMENT 2 : EVERY ACTION HAS A EQUAL AND OPP REACTION
Now many ppl have said that the reason is inertia
But i'm thinking like this please tell me where i am wrong
Consider the cloth and dishes as one system
Because the cloth is being pulled with a certain acceleration(non inertial frame)
pseudo force is valid and that acts on the dishes only to balance both forces.So the dishes don't move
Compare this situation to the famous wedge-block situation don't you consider pseudo force when the wedge move
That's what i felt
Pls notify if i'm wrong in any way
Comments (17)
Yogesh C
Blazing goIITian
Joined: 3 May 2007
Posts: 376
22 May 2007 17:35:15 IST
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pseudo force is not real force so i think u shdnt considr that nd take ground as reference
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22 May 2007 17:58:03 IST
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ok sit on the table itself along with the dishes
which frame of reference are you in now?
how can yyou find the actual force now?
Pseudo force is the only force
the answer is INDEPENDENT of the frame of reference
so if you are on a non inertial frame of reference or an inertial frame the answer's the sam
i think mine shud be the answer but still not sure
experts????
22 May 2007 18:11:21 IST
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@coolyog:
if u r sitting in a car and you experience an outward F (and smash ur face on the windows) when the car is turning right??
WHAT THE HELL IS THAT FORCE???
its centriFUGAL force, which is a pseudo force.
so, pseudo forces are valid in non inertial frames (such as turning car) and IN THAT FRAME, ther ARE REAL!!!!!!!!!!!!
@rahul_12345:
see, in the question 1, it is said "without dislodging the dishes"
is u consider the cloth frame, the dishes are dislodged (moved wrt the cloth). So, i think it is already implied by that, that they r talking about ground frame. (ie. the dishes are dislodged in the cloth frame, so the question of taking that as ur frame is itself wrong in this q)
Once it is ground frame, it is clear na? No pseudo F nothin, so it has to be only inertia...
do u think im correct? yes? oh! then do RATE me...
if u r sitting in a car and you experience an outward F (and smash ur face on the windows) when the car is turning right??
WHAT THE HELL IS THAT FORCE???
its centriFUGAL force, which is a pseudo force.
so, pseudo forces are valid in non inertial frames (such as turning car) and IN THAT FRAME, ther ARE REAL!!!!!!!!!!!!
@rahul_12345:
see, in the question 1, it is said "without dislodging the dishes"
is u consider the cloth frame, the dishes are dislodged (moved wrt the cloth). So, i think it is already implied by that, that they r talking about ground frame. (ie. the dishes are dislodged in the cloth frame, so the question of taking that as ur frame is itself wrong in this q)
Once it is ground frame, it is clear na? No pseudo F nothin, so it has to be only inertia...
do u think im correct? yes? oh! then do RATE me...
22 May 2007 18:30:05 IST
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See what i say is it is an accerelating frame of reference when you are positioned on the dishes
in that case there will be this pseudo force acting on the dishes
since there is no other force present these forces have to be balanced for the system not to move
So in that case the reason has to be true
PLS READ THE QUESTION IT IS MENTIONED AS A ACCELERATING FRAME
coz you are dislodging the cloth alright so u are the cloth's reference frame so obviously it is a accelerating frame of reference
U don't pull the cloth with uniform vel Do you'??????
22 May 2007 18:48:23 IST
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Yeah !! You are right Rahul.
From the frame of reference of table cloth, a Pseudo force is acting on the dishes,because they do not move from ground frame. Very True. But then, what is causing this Pseudo force to act ??
Here you need to understand the concept of Pseudo force. It is not an actual force. It is a consequence of inertia. Inertia tries play it's role to stop the dishes from moving. But in a non inertial frame, a Pseudo force has to be considered in order to explain that inertia on the ground frame. The Pseudo force that a person in an accelerating frame see is originated by the virtue of his being accelerated. Nobody applies it, but it comes side by side with an accelerating frame. Or in order to co-relate different non-inertial frames, the effects of an accelerated frame are explained by introducing an imaginary force, called the Pseudo force.
It's inertia from Ground. And on Table, it's the Pseudo force which originated to explain inertia on the ground. So the basic concept is Inertia only. Pseudo force is only to help us understand a law of Physics in a non accelerating frame as observed from an accelerating frame.
Rate me if you find it useful or knock me back if you are still in trouble.
From the frame of reference of table cloth, a Pseudo force is acting on the dishes,because they do not move from ground frame. Very True. But then, what is causing this Pseudo force to act ??
Here you need to understand the concept of Pseudo force. It is not an actual force. It is a consequence of inertia. Inertia tries play it's role to stop the dishes from moving. But in a non inertial frame, a Pseudo force has to be considered in order to explain that inertia on the ground frame. The Pseudo force that a person in an accelerating frame see is originated by the virtue of his being accelerated. Nobody applies it, but it comes side by side with an accelerating frame. Or in order to co-relate different non-inertial frames, the effects of an accelerated frame are explained by introducing an imaginary force, called the Pseudo force.
It's inertia from Ground. And on Table, it's the Pseudo force which originated to explain inertia on the ground. So the basic concept is Inertia only. Pseudo force is only to help us understand a law of Physics in a non accelerating frame as observed from an accelerating frame.
Rate me if you find it useful or knock me back if you are still in trouble.
22 May 2007 19:01:05 IST
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Are you sure
i kinda disagree with your arguement
Tell me why do you consider pseudo forces
eg: where do centrifugal forces come from they are pseudo forces after all
Caonsider a string rotating in a circular motion
what happenes if centrifugal force wud not act it wudnt have continued it's path
So here pseudo force does balance it
where does it originate from? it comes therby coz the sys in eq
22 May 2007 19:14:57 IST
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see u urself have consodered the cloth plus dishes to be the system ... then whatever force is acting on the cloth is acting on the dishes as well ... then from what point does pseudo force comes ... it will only be there if ur system is considered to be the cloth only ... that is the mistake in ur thinking
22 May 2007 19:59:07 IST
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Ofcourse pseudo force is a force.
It is called pseudo as it is acting in non-inertial frame to make newton's law applicable.
So it is force and we cannot deny that by simply assuming it pseudo as the name is a misnomer and sometimes mislead.
In the case of cloth when suddenly pulled without dislodging the dishes, the reason is enormous force applied or impulse is high (sudden change in momentum) and thus the time of interaction of dishes via surface is ot enough toi dislode the dishes, or we can say that the instantaneous frictional force tends to ZERO.
22 May 2007 22:24:01 IST
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mr last min genius....ur ex is gud but man thats not a force...infact itzz ur experience ur intution wich has made u feel that it is the force...dude if u tie a stone to a thread nd set it into circular motion ...then cut the thread..it will move tangentially...not b coz of the force but b coz of the tangential velocity...ur going by intution read physics frm hcv....
what u experience is not a force...it feels like a force due to tangential velocity...wen a car accelerates...u tend to move backk...dude itz not b coz of the force on u..but b cozof the force on the car that makes u feel u maove back...they r NOT REAL...nd ITZZ NOT A MISNOMER mind IT. okay..everything is nt misnomer in physix..the xplanation wich u gav to rahul is also not rite..
@ rahul..itz just to make physics easy..to solve problems...physics can be studied without taking into account the pseudo forces...all problesm can be solved frm ground ref frame but it b comes tough sumtimes dude....
22 May 2007 22:35:42 IST
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..suppose u r sitting on a body wich is undergoing uniform circular motion.....a force is being experienced by the body towards the center wich produces a change in velocity towards the center(direction of vel changes towards center).....now u sit on it.....if u measure the acc of the body ull find it is zero coz u r also acc with it..it appears stationary to u.....so U APPLY CENTRIFUGAL FORCE(pseudo)...to that particle....nd equate it to tension wich gives actual acc of the particle.....got it??/if not then ask me ill xplain...better read hcv properly.
i hav this much to say..i know im rite.....nd this is my last post here..i acnt xplain more......if ur nt satisfied then ask profs...im a physix geek ....i m confident enough
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