fjee aits - rotation

passage: a rod of mass m n length l is placed vertiaclly on a smooth floor as shown in the fig.. The lower end of the rod is given a negligible slight push towrds left due to which the lower end of the rod moves towards left and collides perfectly inelasticaly with the fixed point P and the rod rotates about the point P. [ l = (2 - √3)m ]

1.) the angular vel of the rod just before its lowest point collide with P is --

a) √48g/13

b) √24g/7

c) √21g/7

d) √20g/13

u can chcek the ans if u get anything...

ans ->ans: b

17 Answers

1
skygirl ·

Vcom=ωl/4 ....(because velocity of lowest point is only along plane..) ----> din get [2] [2] [2] [2]

1
skygirl ·

ohk! thanx :)

1
mkagenius ·

also by instantaeneous center method v=wl/4

33
Abhishek Priyam ·

post added to above (post 9... designer post :P) to complete the soln


now is it clear...
ωl/2cos600=v
v=wl/4..

1
skygirl ·

hey am sry 4 not understanding :P but for last two i have been constantly finding out wrong ans ... n am fedup wid my rusted mind [2]

1
skygirl ·

but y wl/4 ?

33
Abhishek Priyam ·

33
Abhishek Priyam ·

no.. theres torque due to mg...

1
skygirl ·

cant we conserve angular momentum about point P???

mvl/2 = Ipw ??

plz correct if wrong...

1
skygirl ·

2.) the angular velocity of the rod just afetr its lowest point collides with P is :

a) g/4

b) g/2

c) √21g/8

d) √21g/32

33
Abhishek Priyam ·

○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○

Vcom=ωl/4 ....(because velocity of lowest point is only along plane..)

mgl/4(2-√3)=1/2mvcom2+1/2ml2/12ω2...

Put vcom and value of l to get answer√24g/7..

[1]

__________

added later to complete the soln..

now is it clear...
ωl/2cos600=v
v=wl/4..

○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●○●

1
skygirl ·

arey... woh toh kii...

then i am getting ans: √9g/14 [2] [2] [2] [2]

33
Abhishek Priyam ·

mgl(1/2-√3/4)=K.E.

33
Abhishek Priyam ·

Okie i am posting sumthing...in a sec

1
skygirl ·

i have a doubt re :)

33
Abhishek Priyam ·

You have a doubt or this is for others..[7]

1
skygirl ·

ans for 2 >>[d]

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