IIT JEE , AIEEE Forum - Physics , Mechanics

hsbhatt

I can see I missed the answer bcos I forgot that PE_i = mga.

Anyway, can someone please clarify how is angular momentum consreved at the beginning when the bullet hits the block.

ramyani

@ vijeshbhute (5)

or hsbhatt (305)

why don't u elaborate plz !!!!!

hsbhatt

@ramyani: The problem asks for the minimum velocity so that the block does not tip over. In your solution you were attempting to see that the block does not even leave the ground in the first place.

If the velocity is sufficiently high, the block begins to rotate about hinge AB with gravity providing the retarding moment. The extremal condition is when the centre of mass is directly above the hinge. Since the torque is zero, the angular acceleration too is zero at this point. So, if you have angular velocity also become zero, the block cannot tip over.

So now apply the energy equation. The non-conservative forces make no contribution to the equation. At the final point, the KE is zero. The change in potential energy of the block can be calclulated by seeing how much the centre of mass shifts in the vertical direction from y = a to y =

2a (which is where I made a monumental error!).

Only as I noted before, I am not convinced that you can apply conservation of angular momentum at the beginning with gravity watching us. Could any of you please clear this point with your teachers as the text book seems to agree with this interpretation?

Some other clarifications to points raised in this thread: You definitely cannot use momentum conservation, the reaction at the hinge prohibits this. Normal Reaction (of the table ) bid us good bye the second the block took off. At the hinge these forces do no work.

priyesh

@hsbhatt as we have taken the axis as AB so as the block takes of from the ground the normal reaction shifts to the point A so since our axis is AB torque of normal reaction is zero

hsbhatt

the previous post is the answer to which qn of mine?

atavistic

Arey since bullet and block both are in same system.You can imagine this when the bullet just hits the cube, just draw a circle circumscribing them,so then the impulse it gives become internal :)

hsbhatt

Thank you very much for the patronizing reply. I was not wondering about the internal forces, but about gravity. I ask is it enough that at the initial state L = 0 for us to apply conservation of angular momentum? That's all.

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