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Question Number 24739 by Tinkutara last updated on 25/Nov/17
A particle is suspended vertically  from point O by ideal string of length  L. It is given horizontal velocity ′v′.  There is vertical line AB at a distance  (L/8) from P. At some point, it leaves  circular motion and follows projectile  motion. At the instant it crosses AB,  its velocity is horizontal. Find u
AparticleissuspendedverticallyfrompointObyidealstringoflengthL.Itisgivenhorizontalvelocityv.ThereisverticallineABatadistanceL8fromP.Atsomepoint,itleavescircularmotionandfollowsprojectilemotion.AttheinstantitcrossesAB,itsvelocityishorizontal.Findu
Commented by Tinkutara last updated on 25/Nov/17
Commented by ajfour last updated on 25/Nov/17
u=(√(((3(√3)+4)gL)/2)) .
u=(33+4)gL2.
Commented by Tinkutara last updated on 25/Nov/17
How?
How?
Commented by ajfour last updated on 25/Nov/17
Commented by ajfour last updated on 25/Nov/17
(1/2)m(u^2 −v^2 )=mgy=mgL(1+sin θ)                                     ....(i)  ((mv^2 )/L)=mgsin θ  ⇒ v^2 =gLsin θ  ..(ii)  x=(R/2)=((v^2 sin [2(90−θ)])/(2g))=Lcos θ−(L/8)  ⇒gLsin θ×((2sin θcos θ)/(2g))=L(cos θ−(1/8))  ⇒ cos θ(1−cos^2 θ)=cos θ−(1/8)  or  cos θ=(1/2)  Now from (i):   v^2 =(((√3)gL)/2)  using this in (i):  u^2 =v^2 +2gL(1+sin θ)      =(((√3)gL)/2)+2gL(1+((√3)/2))  or   u= (√(((3(√3)+4)gL)/2)) .
12m(u2v2)=mgy=mgL(1+sinθ).(i)mv2L=mgsinθv2=gLsinθ..(ii)x=R2=v2sin[2(90θ)]2g=LcosθL8gLsinθ×2sinθcosθ2g=L(cosθ18)cosθ(1cos2θ)=cosθ18orcosθ=12Nowfrom(i):v2=3gL2usingthisin(i):u2=v2+2gL(1+sinθ)=3gL2+2gL(1+32)or\boldsymbolu=(33+4)\boldsymbolgL2.
Commented by Tinkutara last updated on 25/Nov/17
x=(R/2)=((v^2 sin [2(90−θ)])/(2g))=Lcos θ−(L/8)  How this L(cos θ−(1/8))?
x=R2=v2sin[2(90θ)]2g=LcosθL8HowthisL(cosθ18)?
Commented by ajfour last updated on 25/Nov/17
see diagram, x=Lcos θ−(L/8)  since velocity is horizontal  when as a projectile mass ′m′  crosses AB so from the point  where circular motion ends  and till line AB is reached the  horizontal distance   x=v_x △t=(vsin θ)(((vcos θ)/g)).
seediagram,x=LcosθL8sincevelocityishorizontalwhenasaprojectilemassmcrossesABsofromthepointwherecircularmotionendsandtilllineABisreachedthehorizontaldistance\boldsymbolx=vxt=(vsinθ)(vcosθg).
Commented by Tinkutara last updated on 26/Nov/17
Thank you Sir!
ThankyouSir!
Commented by ajfour last updated on 27/Nov/17
θ, for a reason is acute angled..
θ,forareasonisacuteangled..
Commented by ajfour last updated on 28/Nov/17
mgL(1−cos φ)=(1/2)m(u^2 −v_1 ^2 )  For Tension T to vanish,  mgcos φ=−((mv^2 )/L)  ⇒  φ > 90° .
mgL(1cosϕ)=12m(u2v12)ForTensionTtovanish,mgcosϕ=mv2Lϕ>90°.
Commented by mrW1 last updated on 28/Nov/17
Thank you sir!  Now I understand the question totally.  Can you find the point where the  particle comes back?
Thankyousir!NowIunderstandthequestiontotally.Canyoufindthepointwheretheparticlecomesback?
Commented by mrW1 last updated on 28/Nov/17

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