Q1.A string of bob is attached to a block kept on frictionless surface (bob is hanging ) an impulse is imparted to bob to give it velocity v0 describe motion of block.
33No skygirl not like that.....
Another hint: Let ad (+x direction)be acc of COM of disk and α(In clockwise sense) be its angular acc and ap be acc of plznk(+x direction). For pure rolling......
Acc of point of contact=ad - αR.
Acc of point of contact of plank=ap
ad - αR=ap .........
62This relative motion will cause the condition of pure rolling to be violated!
62for instance,
Assume the ball is at rest and the plank is at rest. And there is no movement or force at all.
There will be pure rolling? (why?) bcos the point of contact has no relative motion!!!
If u started to apply a force on the plank.. then..
tthe plank would move.. but the ball would not have any change in its motion (as u rightly observed!)
62Sky, to add to abhishek's explanation.
The force on the plank will not have any effect on the ball. That is a perfect observation.
But will it effect its pure rolling? YES
Why?
For pure rolling the relative motion between the two bodies is the same!
So, when a force is applied to the plank, there may be additional slip that may come between the two bodies.
62yes, finally i think i got something worth telling! question 1
Total energy will be conserved 1/2 mv2
now the momentum of system will be mv
the only way the top block will move backwards is when teh lower bob will have x component of its momentum at more than mv
so its velocity's x component should be more than mv.
So the kinetic eneryg of the bob will go beyond 1/mv2
This is not possible..
hece correct choice is b!
33Skygirl u should also try this for better understanding of relation b/w accelerations....
62The 2nd question is easire than most of u feel!!
U need to find alpha and a of the ball. And thus the velocity of the point of contact in absolute frame
ANd u need to find the velocity of the plank in absolute frame
They should be equal :)
1wat i think abt the first prob,,,, we should hav been given sum more data... i dunno... sumthing...may be...
1for the second question,
From the thing that….. for pure rolling, acc at the point of contact should be zero,
F2 =0.
And F1 and F3 can have ny value for possibility of pure rolling……
62I thought this question isnt that difficult! still no takers!
But have we reached any conclusion on the first problem?!!
I am still wondering!
33hint for navneet: In pure rolling relative velocity of point of contact is zero same go with acceleration(In layman's language)
@ Navneet
You have a wrong concept of rolling.
I ll put the solution, but in the end.
For a hint : rolling is possible here, just try to recollect the basics & you can easily solve it
@ abhishek
I ll most probably put the solution of first part tonight.
33Oh dear!!! i am afraid this is not the case.
Consider basic definition of pure rolling.........
33Assume whatever u think is necessory, u may take m,m u have to solve physics not maths ki m1, m2 lo......
So last words about the previous question???
33Or for finding that x take friction=0 and then solve using pure rolling... then x will come out to be that...
33for any body(hoolow or solid sphere or disk) (on a stationary surface) if Force F is applied at a distance x above COM such that x=Ic/mR then it will do pure rolling without any frictional help.....
_______________________________________
then force applied will be enough for providing torque for rotaion and force for translation...
try solving taking given x and friction will come out to be zero..
below that friction is backward.. above that x friction will be forward...
1go with psuedo force
the motion of bob wrt block will be like that of pendulum
find the tension
it would be variable and so will be the motion of the block
1I dont think rolling will be possible here.
Because we need friction between surface for pure rolling.
How can there be rolling if μ = 0 ??
33yes tapan is right...
Nishant bhaiya conserved momentum after velocity is imparted.. and sky conserved before and after velocity is imparted...
What Nishant bhaiya said is given in form of fig at top of this page... and last page of earlier page..
21MANI : Sir has conserved momentum after impulse is applied,
earlier sky(mostly) was tellin ther isnt any external force at any pt. in time wich is not da case!
11sir u said in post 9 that
no sky.. there is an impulse..
so u need to consider a lot of other things...
i will give u an example.. try giving a velocity to one end of a string with balls on both ends... will these balls just satrt to move ...
i think abhishek wants smoe more details :)
i mean think a bit more than u have...
but u have solved the ques by conserving momentum in post #47
please clear my dilemma
