|

Mechanics

Blazing goIITian

 Joined: 9 Nov 2006 Post: 814
15 Sep 2007 14:44:54 IST
0 People liked this
45
3452
AIEEE 2007. Conceptual....
Engineering Entrance , Medical Entrance , AIPMT , JEE Main , AIIMS , JEE Advanced , Physics , Mechanics

A ball is dropped from some height on a heap of sand. The ball pireces into the heap and stops after travelling a certain distance.

(a) Only momentum is conserved
(b) Only Kinetic Energy is conserved
(c) Both are conserved
(d) None is conserved

I think answer should be (d) but in solution it is given (A)

Pls. explain........

Cool goIITian

Joined: 15 Aug 2007
Posts: 36
17 Sep 2007 17:37:38 IST
0 people liked this

consider earth + ball system.hence momentum is conserved.

Blazing goIITian

Joined: 13 Jan 2007
Posts: 1289
17 Sep 2007 18:11:45 IST
2 people liked this

arrey guys & gals( if any)

why u people are considering only stone & heap as a system....?????

considering stone + sand + earth as a system i m gonna explain d answers...

starting from 1st option:-

1) momentum is conserved . purav says that stone is accelerating due to gravity.. its perfectly right . when u consider perfectly elastic colloision then why don't we consider gravity.?? because we talk in particular direction.......here also G force is internal force..............as soon as stone will get into the heap earth will also move. in the direction of stone.......( but as mass of earth is very very very very very large its velocity will be very very small . so we can neglect it........

2) energy can be conserved here.. but as soon as stone will get into heap it will produce some heat hence energy is not conserved...

3& 4) cann;t be

hope its clear now

Cool goIITian

Joined: 13 Aug 2007
Posts: 78
17 Sep 2007 18:24:30 IST
0 people liked this

momentum can b conserved just b4 & just aftr a colision

Blazing goIITian

Joined: 9 Nov 2006
Posts: 814
17 Sep 2007 19:55:07 IST
0 people liked this

@ Shubham,

even if we consider earth + stone + ball as a system, then too the frictional force provided by the heap of sand cannot be considered as internal force. It is a dissipative force. Also if it is the case of perfectly inelastic collision then what is the common velocity of com of heap and ball. I mean the com just shifts. Afterwards it will come to rest. So the momentum is finally becoming zero.

@ Swashata,

conservation of energy means only conservetion of KE (in this case) You can alternatively look that option as 'No Work is done'.

Cool goIITian

Joined: 3 Jul 2007
Posts: 46
22 Sep 2007 22:53:42 IST
0 people liked this

i think purav is right ....

bcuzz it can b easily seen dat KE isnt conservd...
abt momentum.. there is a force acting along that axis of motion ( gravity) so i think momentu 2 cant b conserved....

Cool goIITian

Joined: 10 Apr 2007
Posts: 83
23 Sep 2007 09:41:19 IST
0 people liked this

we cannot take...earth as a system because it is of infinite...mass...considering heap and ..stone as a system...as the impulse ....of gravity is ...very less and neglgible...( but i have a doubt...it is not necessarily ...negligible in all cases... so how do we check???) so we can conserve ...it in the vertical direction....but if we consider the system as only ball...then there will be impulse of normal contact force...which has to be taken to account ...so in that case we cant ....take the ...conservation of momentum...

Cool goIITian

Joined: 3 Jul 2007
Posts: 46
23 Sep 2007 22:31:07 IST
0 people liked this

dats wat purav just said my dear.. dat there is force along its direction of motion(even wen in contact with the sand---gravity) which makes it impossible to conserve the momentum...
huge mass or small mass .. u can consider watever u want to b the system.... there is no mass limiti for systems...

Cool goIITian

Joined: 10 Apr 2007
Posts: 83
24 Sep 2007 10:43:58 IST
0 people liked this

ok thanx...but ...one more thing ...i think....if u considering sand and stone as one system its nothing but considering earth in the system rite??? anyways ..he is nt askin us to calculate rite??? so whats the grt deal in takin earth also in our system..??? if we take that there wud be no impulse rite??? so we can conserve ...momentum...rite???

Blazing goIITian

Joined: 9 Nov 2006
Posts: 814
24 Sep 2007 14:48:50 IST
0 people liked this

But even if you take earth in the system then what about the dissipative frictional force provided by the sand......

Cool goIITian

Joined: 10 Apr 2007
Posts: 83
24 Sep 2007 19:38:58 IST
0 people liked this

hey guys i think i got the answer .....
take the sand and stone as a system.... ..so there is no external force acting therefore ...momentum is conserved its as simple ...u see we got a looot confused.....and purav....friction .....force both action and reaction ....act on bodies of the system...so the net of it ..is zero....because it is an internal force...but if u consider any one particle as ur system....then it wud do some work...

if i am rigfht then pls rate me...!! hope its clear...if not then ...pls ask ill try my level best to answer it

Blazing goIITian

Joined: 9 Nov 2006
Posts: 814
25 Sep 2007 16:17:23 IST
0 people liked this

One problem Ksudha_iit

If you take ball + sand as system then gravity will be external force. (Eqarth applies it on the ball. So momentum changes. Work done => Chg in Kinetic energy. Explain)

Cool goIITian

Joined: 10 Apr 2007
Posts: 83
25 Sep 2007 18:10:50 IST
0 people liked this

hey if u take sand in ur system...then its the same thing as taking earth as...in ur system....its heap of sand...rite??? so gravity is internal....so no external force...wat say??

Blazing goIITian

Joined: 9 Nov 2006
Posts: 814
25 Sep 2007 22:44:26 IST
0 people liked this

How can gravity be internal force. Some third object (earth) is exerting it........

Cool goIITian

Joined: 10 Apr 2007
Posts: 83
26 Sep 2007 12:41:22 IST
0 people liked this

fine then take ...earth also in ur system...there wud be no problem...its a conceptual Q rite...u cant solve it......coz the mass of earth is very large.. thn gravity wud be an internal force...fine??

Blazing goIITian

Joined: 9 Nov 2006
Posts: 814
26 Sep 2007 15:17:06 IST
0 people liked this

Okay.. Fine.......

Cool goIITian

Joined: 10 Apr 2007
Posts: 83
27 Sep 2007 20:58:16 IST
0 people liked this

so is the discussion over??? hey post some more gud Q..

Blazing goIITian

Joined: 9 Nov 2006
Posts: 814
27 Sep 2007 22:38:47 IST
0 people liked this

If you have completed electrostatics then there is one more undiscussed (actually half discussed question) in Electricity section under the name 'System of Charges'. If you cant find it there then please visit my profile and search it there.

I am waiting for a satisfactory answer.

Blazing goIITian

Joined: 5 May 2007
Posts: 686
28 Sep 2007 07:26:59 IST
0 people liked this

clearly KE is not conserved
but momentum is conserved
because , the earth alone doesn't pull the ball.
the ball also pulls the earth with equal gravitational force
these two forces nullify and momentum will be conserved
MOMENTUM IS ALWAYS CONSERVED, or NEWTON'S THIRD LAW IS WRONG

Cool goIITian

Joined: 10 Apr 2007
Posts: 83
28 Sep 2007 10:30:28 IST
0 people liked this

huh???? what are you talking about...ur answer is only valid if the system consists of both earth and stone....and for ur ...information....the action and reaction act on different bodies so they cant cancel each other unless and until they become internal forces...

Blazing goIITian

Joined: 5 May 2007
Posts: 686
28 Sep 2007 14:07:49 IST
0 people liked this

gravitational force acting on ball at any instant = GMm/R^2
gravitational force acting on earth at any instant = GMm/R^2

ie, the rate of change of momentum of both bodies is same but in opposite directions

so, momentum is conserved

as there was no reference to the system, we must take the entire thing ( universe) as the system

 Some HTML allowed. Keep your comments above the belt or risk having them deleted. Signup for a avatar to have your pictures show up by your comment If Members see a thread that violates the Posting Rules, bring it to the attention of the Moderator Team

## For Quick Info

Name

Mobile

E-mail

City

Class

Vertical Limit

Top Contributors
All Time This Month Last Week
1. Bipin Dubey
 Altitude - 16545 m Post - 7958
2. Himanshu
 Altitude - 10925 m Post - 3836
3. Hari Shankar
 Altitude - 9960 m Post - 2185
4. edison
 Altitude - 10815 m Post - 7797
5. Sagar Saxena
 Altitude - 8625 m Post - 8064
 Altitude - 6330 m Post - 1979

Physics

Topics

Mathematics

Chemistry

Biology

Institutes

Parents Corner

Board

Fun Zone