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Question Number 17816 by Tinkutara last updated on 11/Jul/17
A man is standing on top of a building  100 m high. He throws two balls  vertically, one at t = 0 and other after  a time interval (less than 2 seconds).  The later ball is thrown at a velocity of  half the first. The vertical gap between  first and second ball is +15 m at t = 2 s.  The gap is found to remain constant.  Calculate the velocity with which the  balls were thrown and the exact time  interval between their throw.
$$\mathrm{A}\:\mathrm{man}\:\mathrm{is}\:\mathrm{standing}\:\mathrm{on}\:\mathrm{top}\:\mathrm{of}\:\mathrm{a}\:\mathrm{building} \\ $$$$\mathrm{100}\:\mathrm{m}\:\mathrm{high}.\:\mathrm{He}\:\mathrm{throws}\:\mathrm{two}\:\mathrm{balls} \\ $$$$\mathrm{vertically},\:\mathrm{one}\:\mathrm{at}\:{t}\:=\:\mathrm{0}\:\mathrm{and}\:\mathrm{other}\:\mathrm{after} \\ $$$$\mathrm{a}\:\mathrm{time}\:\mathrm{interval}\:\left(\mathrm{less}\:\mathrm{than}\:\mathrm{2}\:\mathrm{seconds}\right). \\ $$$$\mathrm{The}\:\mathrm{later}\:\mathrm{ball}\:\mathrm{is}\:\mathrm{thrown}\:\mathrm{at}\:\mathrm{a}\:\mathrm{velocity}\:\mathrm{of} \\ $$$$\mathrm{half}\:\mathrm{the}\:\mathrm{first}.\:\mathrm{The}\:\mathrm{vertical}\:\mathrm{gap}\:\mathrm{between} \\ $$$$\mathrm{first}\:\mathrm{and}\:\mathrm{second}\:\mathrm{ball}\:\mathrm{is}\:+\mathrm{15}\:\mathrm{m}\:\mathrm{at}\:{t}\:=\:\mathrm{2}\:\mathrm{s}. \\ $$$$\mathrm{The}\:\mathrm{gap}\:\mathrm{is}\:\mathrm{found}\:\mathrm{to}\:\mathrm{remain}\:\mathrm{constant}. \\ $$$$\mathrm{Calculate}\:\mathrm{the}\:\mathrm{velocity}\:\mathrm{with}\:\mathrm{which}\:\mathrm{the} \\ $$$$\mathrm{balls}\:\mathrm{were}\:\mathrm{thrown}\:\mathrm{and}\:\mathrm{the}\:\mathrm{exact}\:\mathrm{time} \\ $$$$\mathrm{interval}\:\mathrm{between}\:\mathrm{their}\:\mathrm{throw}. \\ $$
Answered by mrW1 last updated on 11/Jul/17
h_1 =v_1 t+(1/2)gt^2   h_2 =(1/2)v_1 (t−Δt)+(1/2)g(t−Δt)^2   Δh=h_1 −h_2 =(1/2)v_1 (t+Δt)+(1/2)g(2t−Δt)Δt  =(1/2)v_1 t+gtΔt+(1/2)v_1 Δt−(1/2)gΔt^2   =t((1/2)v_1 +gΔt)+(1/2)Δt(v_1 −gΔt)  since Δh remains constant  ⇒(1/2)v_1 +gΔt=0  ⇒v_1 =−2gΔt  Δh=(1/2)Δt(v_1 −gΔt)=−(3/2)gΔt  −15=−(3/2)gΔt  ⇒Δt=1 s  ⇒v_1 =−2g×1=−20 m/s (⇈)
$$\mathrm{h}_{\mathrm{1}} =\mathrm{v}_{\mathrm{1}} \mathrm{t}+\frac{\mathrm{1}}{\mathrm{2}}\mathrm{gt}^{\mathrm{2}} \\ $$$$\mathrm{h}_{\mathrm{2}} =\frac{\mathrm{1}}{\mathrm{2}}\mathrm{v}_{\mathrm{1}} \left(\mathrm{t}−\Delta\mathrm{t}\right)+\frac{\mathrm{1}}{\mathrm{2}}\mathrm{g}\left(\mathrm{t}−\Delta\mathrm{t}\right)^{\mathrm{2}} \\ $$$$\Delta\mathrm{h}=\mathrm{h}_{\mathrm{1}} −\mathrm{h}_{\mathrm{2}} =\frac{\mathrm{1}}{\mathrm{2}}\mathrm{v}_{\mathrm{1}} \left(\mathrm{t}+\Delta\mathrm{t}\right)+\frac{\mathrm{1}}{\mathrm{2}}\mathrm{g}\left(\mathrm{2t}−\Delta\mathrm{t}\right)\Delta\mathrm{t} \\ $$$$=\frac{\mathrm{1}}{\mathrm{2}}\mathrm{v}_{\mathrm{1}} \mathrm{t}+\mathrm{gt}\Delta\mathrm{t}+\frac{\mathrm{1}}{\mathrm{2}}\mathrm{v}_{\mathrm{1}} \Delta\mathrm{t}−\frac{\mathrm{1}}{\mathrm{2}}\mathrm{g}\Delta\mathrm{t}^{\mathrm{2}} \\ $$$$=\mathrm{t}\left(\frac{\mathrm{1}}{\mathrm{2}}\mathrm{v}_{\mathrm{1}} +\mathrm{g}\Delta\mathrm{t}\right)+\frac{\mathrm{1}}{\mathrm{2}}\Delta\mathrm{t}\left(\mathrm{v}_{\mathrm{1}} −\mathrm{g}\Delta\mathrm{t}\right) \\ $$$$\mathrm{since}\:\Delta\mathrm{h}\:\mathrm{remains}\:\mathrm{constant} \\ $$$$\Rightarrow\frac{\mathrm{1}}{\mathrm{2}}\mathrm{v}_{\mathrm{1}} +\mathrm{g}\Delta\mathrm{t}=\mathrm{0} \\ $$$$\Rightarrow\mathrm{v}_{\mathrm{1}} =−\mathrm{2g}\Delta\mathrm{t} \\ $$$$\Delta\mathrm{h}=\frac{\mathrm{1}}{\mathrm{2}}\Delta\mathrm{t}\left(\mathrm{v}_{\mathrm{1}} −\mathrm{g}\Delta\mathrm{t}\right)=−\frac{\mathrm{3}}{\mathrm{2}}\mathrm{g}\Delta\mathrm{t} \\ $$$$−\mathrm{15}=−\frac{\mathrm{3}}{\mathrm{2}}\mathrm{g}\Delta\mathrm{t} \\ $$$$\Rightarrow\Delta\mathrm{t}=\mathrm{1}\:\mathrm{s} \\ $$$$\Rightarrow\mathrm{v}_{\mathrm{1}} =−\mathrm{2g}×\mathrm{1}=−\mathrm{20}\:\mathrm{m}/\mathrm{s}\:\left(\upuparrows\right) \\ $$
Commented by Tinkutara last updated on 11/Jul/17
Thanks Sir!
$$\mathrm{Thanks}\:\mathrm{Sir}! \\ $$
Answered by ajfour last updated on 11/Jul/17
Assuming, balls are thrown   vertically upwards. First ball at a  velocity 2u, and second after time   t_0  at velocity u.  if  t>t_0  we have   s_1 −s_2 =15     (s_1 −s_2 =−15 not possible)   [2ut−(1/2)gt^2 ]−[u(t−t_0 )−(1/2)g(t−t_0 )^2 ]=15  ⇒   u(t+t_0 )−(g/2)t_0 (2t−t_0 )−15=0   since this is true  for all times t>t_0   until any ball hits the ground, so   coefficient of t in equation and the  constant term are both zero.   u−gt_0 =0    and    ut_0 +((gt_0 ^2 )/2)−15=0    or  u=gt_0  and substituting this in  second equation, we get      ((3gt_0 ^2 )/2)=15   ⇒    t_0 =(√((10)/g))        u=gt_0  =(√(10g))  if  g=10m/s^2 , t_0 =1s and u=10m/s.
$$\mathrm{Assuming},\:\mathrm{balls}\:\mathrm{are}\:\mathrm{thrown}\: \\ $$$$\mathrm{vertically}\:\mathrm{upwards}.\:\mathrm{First}\:\mathrm{ball}\:\mathrm{at}\:\mathrm{a} \\ $$$$\mathrm{velocity}\:\mathrm{2u},\:\mathrm{and}\:\mathrm{second}\:\mathrm{after}\:\mathrm{time} \\ $$$$\:\mathrm{t}_{\mathrm{0}} \:\mathrm{at}\:\mathrm{velocity}\:\mathrm{u}. \\ $$$$\mathrm{if}\:\:\mathrm{t}>\mathrm{t}_{\mathrm{0}} \:\mathrm{we}\:\mathrm{have} \\ $$$$\:\mathrm{s}_{\mathrm{1}} −\mathrm{s}_{\mathrm{2}} =\mathrm{15}\:\:\:\:\:\left(\mathrm{s}_{\mathrm{1}} −\mathrm{s}_{\mathrm{2}} =−\mathrm{15}\:\mathrm{not}\:\mathrm{possible}\right) \\ $$$$\:\left[\mathrm{2ut}−\frac{\mathrm{1}}{\mathrm{2}}\mathrm{gt}^{\mathrm{2}} \right]−\left[\mathrm{u}\left(\mathrm{t}−\mathrm{t}_{\mathrm{0}} \right)−\frac{\mathrm{1}}{\mathrm{2}}\mathrm{g}\left(\mathrm{t}−\mathrm{t}_{\mathrm{0}} \right)^{\mathrm{2}} \right]=\mathrm{15} \\ $$$$\Rightarrow\:\:\:\mathrm{u}\left(\mathrm{t}+\mathrm{t}_{\mathrm{0}} \right)−\frac{\mathrm{g}}{\mathrm{2}}\mathrm{t}_{\mathrm{0}} \left(\mathrm{2t}−\mathrm{t}_{\mathrm{0}} \right)−\mathrm{15}=\mathrm{0} \\ $$$$\:\mathrm{since}\:\mathrm{this}\:\mathrm{is}\:\mathrm{true}\:\:\mathrm{for}\:\mathrm{all}\:\mathrm{times}\:\mathrm{t}>\mathrm{t}_{\mathrm{0}} \\ $$$$\mathrm{until}\:\mathrm{any}\:\mathrm{ball}\:\mathrm{hits}\:\mathrm{the}\:\mathrm{ground},\:\mathrm{so} \\ $$$$\:\mathrm{coefficient}\:\mathrm{of}\:\boldsymbol{\mathrm{t}}\:\mathrm{in}\:\mathrm{equation}\:\mathrm{and}\:\mathrm{the} \\ $$$$\mathrm{constant}\:\mathrm{term}\:\mathrm{are}\:\mathrm{both}\:\mathrm{zero}. \\ $$$$\:\mathrm{u}−\mathrm{gt}_{\mathrm{0}} =\mathrm{0}\:\:\:\:\mathrm{and}\:\:\:\:\mathrm{ut}_{\mathrm{0}} +\frac{\mathrm{gt}_{\mathrm{0}} ^{\mathrm{2}} }{\mathrm{2}}−\mathrm{15}=\mathrm{0} \\ $$$$\:\:\mathrm{or}\:\:\mathrm{u}=\mathrm{gt}_{\mathrm{0}} \:\mathrm{and}\:\mathrm{substituting}\:\mathrm{this}\:\mathrm{in} \\ $$$$\mathrm{second}\:\mathrm{equation},\:\mathrm{we}\:\mathrm{get} \\ $$$$\:\:\:\:\frac{\mathrm{3gt}_{\mathrm{0}} ^{\mathrm{2}} }{\mathrm{2}}=\mathrm{15}\:\:\:\Rightarrow\:\:\:\:\mathrm{t}_{\mathrm{0}} =\sqrt{\frac{\mathrm{10}}{\mathrm{g}}}\: \\ $$$$\:\:\:\:\:\mathrm{u}=\mathrm{gt}_{\mathrm{0}} \:=\sqrt{\mathrm{10g}} \\ $$$$\mathrm{if}\:\:\mathrm{g}=\mathrm{10m}/\mathrm{s}^{\mathrm{2}} ,\:\mathrm{t}_{\mathrm{0}} =\mathrm{1s}\:\mathrm{and}\:\mathrm{u}=\mathrm{10m}/\mathrm{s}. \\ $$
Commented by Tinkutara last updated on 11/Jul/17
Thanks Sir!
$$\mathrm{Thanks}\:\mathrm{Sir}! \\ $$

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