Question Number 139167 by Dwaipayan Shikari last updated on 23/Apr/21
Answered by mr W last updated on 23/Apr/21
Commented by mr W last updated on 23/Apr/21
$${y}=\frac{{x}^{\mathrm{2}} }{\mathrm{2}{a}} \\ $$$$\mathrm{tan}\:\theta={y}'=\frac{{x}}{{a}}=\lambda,\:{say} \\ $$$${y}''=\frac{\mathrm{1}}{{a}} \\ $$$${r}=\frac{\left(\mathrm{1}+{y}'^{\mathrm{2}} \right)^{\mathrm{3}/\mathrm{2}} }{{y}''}={a}\left(\mathrm{1}+\frac{{x}^{\mathrm{2}} }{{a}^{\mathrm{2}} }\right)^{\mathrm{3}/\mathrm{2}} ={a}\left(\mathrm{1}+\lambda^{\mathrm{2}} \right)^{\frac{\mathrm{3}}{\mathrm{2}}} \\ $$$${u}={velocity}\:{at}\:\left({x},{y}\right) \\ $$$$\frac{\mathrm{1}}{\mathrm{2}}{mv}^{\mathrm{2}} =\frac{\mathrm{1}}{\mathrm{2}}{mu}^{\mathrm{2}} +{mgy} \\ $$$$\Rightarrow{u}^{\mathrm{2}} ={v}^{\mathrm{2}} −\frac{{gx}^{\mathrm{2}} }{{a}}={v}^{\mathrm{2}} −{ag}\lambda^{\mathrm{2}} \\ $$$${at}\:{highest}\:{position}\:{u}=\mathrm{0}: \\ $$$${v}^{\mathrm{2}} −{ag}\lambda_{{max}} ^{\mathrm{2}} =\mathrm{0} \\ $$$$\Rightarrow\lambda_{{max}} =\frac{{v}}{\:\sqrt{{ag}}} \\ $$$${R}−{mg}\mathrm{cos}\:\theta=\frac{{mu}^{\mathrm{2}} }{{r}}=\frac{{m}\left({v}^{\mathrm{2}} −{ag}\lambda^{\mathrm{2}} \right)}{{a}\left(\mathrm{1}+\lambda^{\mathrm{2}} \right)^{\frac{\mathrm{3}}{\mathrm{2}}} } \\ $$$${R}=\frac{{mg}}{\:\sqrt{\mathrm{1}+{t}^{\mathrm{2}} }}+\frac{{mg}\left(\frac{{v}^{\mathrm{2}} }{{ag}}−\lambda^{\mathrm{2}} \right)}{\left(\mathrm{1}+\lambda^{\mathrm{2}} \right)^{\frac{\mathrm{3}}{\mathrm{2}}} }=\frac{{mg}\left(\mathrm{1}+\frac{{v}^{\mathrm{2}} }{{ag}}\right)}{\:\left(\mathrm{1}+\lambda^{\mathrm{2}} \right)^{\frac{\mathrm{3}}{\mathrm{2}}} } \\ $$$${such}\:{that}\:{the}\:{block}\:{doesn}'{t}\:{slip}, \\ $$$${R}\:\mathrm{sin}\:\theta\leqslant\mu\left({Mg}+{R}\:\mathrm{cos}\:\theta\right) \\ $$$${R}\:\left(\mathrm{sin}\:\theta−\mu\:\mathrm{cos}\:\theta\right)\leqslant\mu{Mg} \\ $$$$\frac{{m}\left(\mathrm{1}+\frac{{v}^{\mathrm{2}} }{{ag}}\right)\left(\lambda−\mu\right)}{\:\left(\mathrm{1}+\lambda^{\mathrm{2}} \right)^{\mathrm{2}} }\leqslant\mu{M} \\ $$$$\frac{\mu{M}}{{m}\left(\mathrm{1}+\frac{{v}^{\mathrm{2}} }{{ag}}\right)}\geqslant\frac{\lambda−\mu}{\:\left(\mathrm{1}+\lambda^{\mathrm{2}} \right)^{\mathrm{2}} }={f}\left(\lambda\right)\:{say} \\ $$$${for}\:{maximum}\:{f}\left(\lambda\right): \\ $$$$\frac{{df}\left(\lambda\right)}{{d}\lambda}=\frac{\mathrm{1}}{\left(\mathrm{1}+\lambda^{\mathrm{2}} \right)^{\mathrm{2}} }−\frac{\mathrm{4}\left(\lambda−\mu\right)\lambda}{\left(\mathrm{1}+\lambda^{\mathrm{2}} \right)^{\mathrm{3}} }=\mathrm{0} \\ $$$$\frac{\mathrm{4}\left(\lambda−\mu\right)\lambda}{\mathrm{1}+\lambda^{\mathrm{2}} }=\mathrm{1} \\ $$$$\mathrm{3}\lambda^{\mathrm{2}} −\mathrm{4}\mu\lambda−\mathrm{1}=\mathrm{0} \\ $$$$\Rightarrow\lambda=\frac{\mathrm{2}\mu+\sqrt{\mathrm{4}\mu^{\mathrm{2}} +\mathrm{3}}}{\mathrm{3}} \\ $$$${with}\:\mu=\frac{\mathrm{1}}{\mathrm{2}} \\ $$$$\Rightarrow\lambda=\frac{\mathrm{2}×\frac{\mathrm{1}}{\mathrm{2}}+\sqrt{\mathrm{4}×\frac{\mathrm{1}}{\mathrm{4}}+\mathrm{3}}}{\mathrm{3}}=\mathrm{1} \\ $$$${f}\left(\lambda\right)_{{max}} =\frac{\mathrm{1}−\frac{\mathrm{1}}{\mathrm{2}}}{\left(\mathrm{1}+\mathrm{1}\right)^{\mathrm{2}} }=\frac{\mathrm{1}}{\mathrm{8}} \\ $$$$\frac{\mu{M}}{{m}\left(\mathrm{1}+\frac{{v}^{\mathrm{2}} }{{ag}}\right)}\geqslant\frac{\mathrm{1}}{\mathrm{8}} \\ $$$$\left(\frac{\mathrm{4}{M}−{m}}{{m}}\right){ag}\geqslant\frac{{v}^{\mathrm{2}} }{{ag}} \\ $$$${v}\leqslant\sqrt{\frac{\mathrm{4}{M}−{m}}{{m}}{ag}} \\ $$$$\lambda_{{max}} =\frac{{v}}{\:\sqrt{{ag}}}=\sqrt{\mathrm{4}×\frac{{M}}{{m}}−\mathrm{1}}>\sqrt{\mathrm{4}×\frac{\mathrm{1}}{\mathrm{2}}−\mathrm{1}}=\mathrm{1} \\ $$$${that}\:{means}\:\lambda=\mathrm{1}\:{is}\:{reached}\:{before}\:{the} \\ $$$${highest}\:{position}\:{is}\:{reached}. \\ $$$${i}.{e}.\:{v}_{{max}} =\sqrt{\frac{\mathrm{4}{M}−{m}}{{m}}{ag}}\:{is}\:{valid}. \\ $$$$\Rightarrow\alpha=\mathrm{4},\:\beta=\mathrm{1},\:\gamma=\mathrm{1} \\ $$$$\Rightarrow\alpha+\beta+\gamma=\mathrm{6} \\ $$
Commented by Dwaipayan Shikari last updated on 23/Apr/21
$${Great}\:{sir}\:!\:{But}\:{i}\:{need}\:{a}\:{suggestion}.\:{How}\:{do}\:{you}\:{think}\:{this}\:{kind} \\ $$$${solutions}\:\:{easily}? \\ $$
Commented by mr W last updated on 24/Apr/21
$${it}'{s}\:{not}\:{easy}\:{to}\:{give}\:{a}\:{general}\: \\ $$$${sugestion}.\:{actually}\:{this}\:{is}\:{not}\:{an} \\ $$$${easy}\:{question},\:{because}\:{one}\:{needs}\:{to} \\ $$$${apply}\:{alot}\:{of}\:{things}\:{both}\:{in}\:{physics} \\ $$$${and}\:{in}\:{calculus}. \\ $$
Commented by peter frank last updated on 24/Apr/21
$${GOD}\:\:{has}\:{bless}\:{him}\:{with} \\ $$$${great}\:\:{ability}\:{maths}\:\:{and}\:{physics}. \\ $$
Commented by otchereabdullai@gmail.com last updated on 24/Apr/21
$$\mathrm{Is}\:\mathrm{very}\:\mathrm{true}\:\mathrm{sir}\:\mathrm{Frank}\:\mathrm{i}\:\mathrm{really}\:\mathrm{love}\:\mathrm{this} \\ $$$$\mathrm{man}.\:\mathrm{For}\:\mathrm{me}\:\mathrm{he}\:\mathrm{is}\:\mathrm{the}\:\mathrm{world}\:\mathrm{best}!\: \\ $$$$\mathrm{Because}\:\mathrm{he}\:\mathrm{looks}\:\mathrm{very}\:\mathrm{different}\:\mathrm{in}\: \\ $$$$\mathrm{solving}\:\mathrm{mathematics}\:\mathrm{and}\:\mathrm{physics}\: \\ $$$$\mathrm{questions}.\:\mathrm{And}\:\mathrm{he}\:\mathrm{has}\:\mathrm{idea}\:\mathrm{in}\:\mathrm{almost} \\ $$$$\mathrm{all}\:\mathrm{questions}\:\mathrm{even}\:\mathrm{if}\:\mathrm{the}\:\mathrm{question} \\ $$$$\mathrm{is}\:\mathrm{wrong}\:\mathrm{he}\:\mathrm{can}\:\mathrm{tell}\:\mathrm{you}\:\mathrm{the}\:\mathrm{quetion}\:\mathrm{is} \\ $$$$\mathrm{wrong}\:\mathrm{and}\:\mathrm{tell}\:\mathrm{the}\:\mathrm{reason}\:\mathrm{and}\:\mathrm{how}\:\mathrm{the} \\ $$$$\mathrm{question}\:\mathrm{should}\:\mathrm{look}\:\mathrm{like}\:\mathrm{even}\:\mathrm{if}\:\mathrm{is} \\ $$$$\mathrm{a}\:\mathrm{very}\:\mathrm{difficult}\:\mathrm{question}!\:\boldsymbol{\mathrm{I}}\:\boldsymbol{\mathrm{wish}}\:\boldsymbol{\mathrm{he}}\:\boldsymbol{\mathrm{is}} \\ $$$$\boldsymbol{\mathrm{my}}\:\boldsymbol{\mathrm{father}}!\:\mathrm{God}\:\mathrm{bless}\:\mathrm{you}\:\boldsymbol{\mathrm{prof}}\:\boldsymbol{\mathrm{W}}\:!\:\boldsymbol{\mathrm{long}} \\ $$$$\boldsymbol{\mathrm{life}}\:\boldsymbol{\mathrm{is}}\:\boldsymbol{\mathrm{what}}\:\boldsymbol{\mathrm{we}}\:\boldsymbol{\mathrm{pray}}\:\boldsymbol{\mathrm{for}}\:\boldsymbol{\mathrm{you}}\:! \\ $$
Commented by mr W last updated on 24/Apr/21
$${thanks}\:{to}\:{you}\:{both}\:{for}\:{your}\:{heart}− \\ $$$${warming}\:{words}! \\ $$