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Question Number 18131 by Tinkutara last updated on 15/Jul/17

A stone is projected from a point on the  ground in such a direction so as to hit a  bird on the top of a telegraph post of  height h, and then attain a height 2h  above the ground. If, at an instant of  projection, the bird were to fly away  horizontal with a uniform speed, find  the ratio of the horizontal velocities of  the bird and the stone, if the stone still  hits the bird.

$$\mathrm{A}\:\mathrm{stone}\:\mathrm{is}\:\mathrm{projected}\:\mathrm{from}\:\mathrm{a}\:\mathrm{point}\:\mathrm{on}\:\mathrm{the} \\ $$$$\mathrm{ground}\:\mathrm{in}\:\mathrm{such}\:\mathrm{a}\:\mathrm{direction}\:\mathrm{so}\:\mathrm{as}\:\mathrm{to}\:\mathrm{hit}\:\mathrm{a} \\ $$$$\mathrm{bird}\:\mathrm{on}\:\mathrm{the}\:\mathrm{top}\:\mathrm{of}\:\mathrm{a}\:\mathrm{telegraph}\:\mathrm{post}\:\mathrm{of} \\ $$$$\mathrm{height}\:{h},\:\mathrm{and}\:\mathrm{then}\:\mathrm{attain}\:\mathrm{a}\:\mathrm{height}\:\mathrm{2}{h} \\ $$$$\mathrm{above}\:\mathrm{the}\:\mathrm{ground}.\:\mathrm{If},\:\mathrm{at}\:\mathrm{an}\:\mathrm{instant}\:\mathrm{of} \\ $$$$\mathrm{projection},\:\mathrm{the}\:\mathrm{bird}\:\mathrm{were}\:\mathrm{to}\:\mathrm{fly}\:\mathrm{away} \\ $$$$\mathrm{horizontal}\:\mathrm{with}\:\mathrm{a}\:\mathrm{uniform}\:\mathrm{speed},\:\mathrm{find} \\ $$$$\mathrm{the}\:\mathrm{ratio}\:\mathrm{of}\:\mathrm{the}\:\mathrm{horizontal}\:\mathrm{velocities}\:\mathrm{of} \\ $$$$\mathrm{the}\:\mathrm{bird}\:\mathrm{and}\:\mathrm{the}\:\mathrm{stone},\:\mathrm{if}\:\mathrm{the}\:\mathrm{stone}\:\mathrm{still} \\ $$$$\mathrm{hits}\:\mathrm{the}\:\mathrm{bird}. \\ $$

Commented by Tinkutara last updated on 15/Jul/17

Commented by ajfour last updated on 15/Jul/17

y=xtan θ−((gx^2 )/(2u^2 cos^2 θ))  let m=tan θ  , θ the∠ of projection.  x=d, and and x=R−d  when y=h   d is the distance to telegraph post  from point of projection. R is the  range of stone.So  h=mx−((gx^2 )/2)(1+m^2 )  g(1+m^2 )x^2 −2mx+2h=0  roots are x=d  and  x=R−d  ⇒ sum of roots=R=((2m)/(g(1+m^2 )))  product = d(R−d)=((2h)/(g(1+m^2 )))  ⇒((d(R−d))/R)=(h/m)       ....(i)  2h=((u^2 sin^2 θ)/(2g))  and R=((2u^2 sin θcos θ)/g)  dividing equations we get     ((8h)/R)=m  ⇒    (h/m)=(R/8)  substituting for (h/m)  in (i)  ((d(R−d))/R)=(R/8)  ⇒   (d/R)(1−(d/R))=(1/8)  ((d/R))^2 −((d/R))+(1/8)=0  ⇒   ((d/R)−(1/2))^2 =(1/8)   ⇒  R−2d=(R/(√2))  R(1−(1/(√2)))=2d  let v be the bird velocity  (v/(ucos θ))=((R−2d)/(R−d))=((1−((2d)/R))/(1−d/R))              =((1−(1−(1/(√2))))/(1−((1/2)−(1/(2(√2))))))=((((1/(√2))))/((1/2)(1+(1/(√2)))))             =(2/((√2)+1))  .

$$\mathrm{y}=\mathrm{xtan}\:\theta−\frac{\mathrm{gx}^{\mathrm{2}} }{\mathrm{2u}^{\mathrm{2}} \mathrm{cos}\:^{\mathrm{2}} \theta} \\ $$$$\mathrm{let}\:\mathrm{m}=\mathrm{tan}\:\theta\:\:,\:\theta\:\mathrm{the}\angle\:\mathrm{of}\:\mathrm{projection}. \\ $$$$\mathrm{x}=\mathrm{d},\:\mathrm{and}\:\mathrm{and}\:\mathrm{x}=\mathrm{R}−\mathrm{d}\:\:\mathrm{when}\:\mathrm{y}=\mathrm{h} \\ $$$$\:\mathrm{d}\:\mathrm{is}\:\mathrm{the}\:\mathrm{distance}\:\mathrm{to}\:\mathrm{telegraph}\:\mathrm{post} \\ $$$$\mathrm{from}\:\mathrm{point}\:\mathrm{of}\:\mathrm{projection}.\:\mathrm{R}\:\mathrm{is}\:\mathrm{the} \\ $$$$\mathrm{range}\:\mathrm{of}\:\mathrm{stone}.\mathrm{So} \\ $$$$\mathrm{h}=\mathrm{mx}−\frac{\mathrm{gx}^{\mathrm{2}} }{\mathrm{2}}\left(\mathrm{1}+\mathrm{m}^{\mathrm{2}} \right) \\ $$$$\mathrm{g}\left(\mathrm{1}+\mathrm{m}^{\mathrm{2}} \right)\mathrm{x}^{\mathrm{2}} −\mathrm{2mx}+\mathrm{2h}=\mathrm{0} \\ $$$$\mathrm{roots}\:\mathrm{are}\:\mathrm{x}=\mathrm{d}\:\:\mathrm{and}\:\:\mathrm{x}=\mathrm{R}−\mathrm{d} \\ $$$$\Rightarrow\:\mathrm{sum}\:\mathrm{of}\:\mathrm{roots}=\mathrm{R}=\frac{\mathrm{2m}}{\mathrm{g}\left(\mathrm{1}+\mathrm{m}^{\mathrm{2}} \right)} \\ $$$$\mathrm{product}\:=\:\mathrm{d}\left(\mathrm{R}−\mathrm{d}\right)=\frac{\mathrm{2h}}{\mathrm{g}\left(\mathrm{1}+\mathrm{m}^{\mathrm{2}} \right)} \\ $$$$\Rightarrow\frac{\mathrm{d}\left(\mathrm{R}−\mathrm{d}\right)}{\mathrm{R}}=\frac{\mathrm{h}}{\mathrm{m}}\:\:\:\:\:\:\:....\left(\mathrm{i}\right) \\ $$$$\mathrm{2h}=\frac{\mathrm{u}^{\mathrm{2}} \mathrm{sin}\:^{\mathrm{2}} \theta}{\mathrm{2g}}\:\:\mathrm{and}\:\mathrm{R}=\frac{\mathrm{2u}^{\mathrm{2}} \mathrm{sin}\:\theta\mathrm{cos}\:\theta}{\mathrm{g}} \\ $$$$\mathrm{dividing}\:\mathrm{equations}\:\mathrm{we}\:\mathrm{get} \\ $$$$\:\:\:\frac{\mathrm{8h}}{\mathrm{R}}=\mathrm{m}\:\:\Rightarrow\:\:\:\:\frac{\mathrm{h}}{\mathrm{m}}=\frac{\mathrm{R}}{\mathrm{8}} \\ $$$$\mathrm{substituting}\:\mathrm{for}\:\frac{\mathrm{h}}{\mathrm{m}}\:\:\mathrm{in}\:\left(\mathrm{i}\right) \\ $$$$\frac{\mathrm{d}\left(\mathrm{R}−\mathrm{d}\right)}{\mathrm{R}}=\frac{\mathrm{R}}{\mathrm{8}} \\ $$$$\Rightarrow\:\:\:\frac{\mathrm{d}}{\mathrm{R}}\left(\mathrm{1}−\frac{\mathrm{d}}{\mathrm{R}}\right)=\frac{\mathrm{1}}{\mathrm{8}} \\ $$$$\left(\frac{\mathrm{d}}{\mathrm{R}}\right)^{\mathrm{2}} −\left(\frac{\mathrm{d}}{\mathrm{R}}\right)+\frac{\mathrm{1}}{\mathrm{8}}=\mathrm{0} \\ $$$$\Rightarrow\:\:\:\left(\frac{\mathrm{d}}{\mathrm{R}}−\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{2}} =\frac{\mathrm{1}}{\mathrm{8}}\:\:\:\Rightarrow\:\:\mathrm{R}−\mathrm{2d}=\frac{\mathrm{R}}{\sqrt{\mathrm{2}}} \\ $$$$\mathrm{R}\left(\mathrm{1}−\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\right)=\mathrm{2d} \\ $$$$\mathrm{let}\:\mathrm{v}\:\mathrm{be}\:\mathrm{the}\:\mathrm{bird}\:\mathrm{velocity} \\ $$$$\frac{\mathrm{v}}{\mathrm{ucos}\:\theta}=\frac{\mathrm{R}−\mathrm{2d}}{\mathrm{R}−\mathrm{d}}=\frac{\mathrm{1}−\frac{\mathrm{2d}}{\mathrm{R}}}{\mathrm{1}−\mathrm{d}/\mathrm{R}} \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:=\frac{\mathrm{1}−\left(\mathrm{1}−\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\right)}{\mathrm{1}−\left(\frac{\mathrm{1}}{\mathrm{2}}−\frac{\mathrm{1}}{\mathrm{2}\sqrt{\mathrm{2}}}\right)}=\frac{\left(\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\right)}{\frac{\mathrm{1}}{\mathrm{2}}\left(\mathrm{1}+\frac{\mathrm{1}}{\sqrt{\mathrm{2}}}\right)} \\ $$$$\:\:\:\:\:\:\:\:\:\:\:=\frac{\mathrm{2}}{\sqrt{\mathrm{2}}+\mathrm{1}}\:\:. \\ $$

Commented by Tinkutara last updated on 15/Jul/17

Thanks Sir!

$$\mathrm{Thanks}\:\mathrm{Sir}! \\ $$

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