Question Number 43585 by ajfour last updated on 12/Sep/18
Commented by ajfour last updated on 12/Sep/18
$${The}\:{small}\:{ball}\:{has}\:{radius}\:\boldsymbol{{r}},\: \\ $$$${while}\:{the}\:{larger}\:{ball}\:{has}\:{radius} \\ $$$$\boldsymbol{{R}},\:{they}\:{collide}\:{on}\:{smooth} \\ $$$${horizontal}\:{ground},\:{impact} \\ $$$${parameter}\:{is}\:{equal}\:{to}\:\boldsymbol{{r}}. \\ $$$${The}\:{small}\:{ball}\:{hits}\:{the}\:{larger} \\ $$$${ball}\:\left({at}\:{rest}\:{initially}\right)\:{with} \\ $$$${speed}\:\boldsymbol{{u}}. \\ $$$$\boldsymbol{{Find}}\:\boldsymbol{\theta},\:\boldsymbol{\phi},\:\boldsymbol{{v}},\:{and}\:\boldsymbol{{V}}\:\:{in}\:{terms}\:{of} \\ $$$$\:\boldsymbol{{m}},\:\boldsymbol{{M}},\:\boldsymbol{{r}},\:\boldsymbol{{R}}\:. \\ $$$$\left({collision}\:{is}\:{completly}\:{elastic}\right). \\ $$
Commented by MrW3 last updated on 12/Sep/18
$${Now}\:{I}\:\:{know}\:{you}\:{are}\:{a}\:{curling}\:{sport} \\ $$$${fan}. \\ $$
Answered by MrW3 last updated on 14/Sep/18
$$\theta=\mathrm{sin}^{−\mathrm{1}} \frac{{r}}{{r}+{R}} \\ $$$$\lambda=\frac{{M}}{{m}} \\ $$$${mu}={mv}\:\mathrm{cos}\:\phi+{MV}\:\:\mathrm{cos}\:\theta \\ $$$$\Rightarrow{u}={v}\:\mathrm{cos}\:\phi+\lambda{V}\:\:\mathrm{cos}\:\theta\:\:\:…\left({i}\right) \\ $$$$\mathrm{0}={mv}\:\mathrm{sin}\:\phi−{MV}\:\:\mathrm{sin}\:\theta \\ $$$$\Rightarrow{v}\:\mathrm{sin}\:\phi=\lambda{V}\:\:\mathrm{sin}\:\theta\:\:…\left({ii}\right) \\ $$$${V}\:−{v}\:\mathrm{cos}\:\left(\phi+\theta\right)={eu}\:\mathrm{cos}\:\theta\:\:…\left({iii}\right) \\ $$$$ \\ $$$${from}\:\left({iii}\right): \\ $$$${V}−{v}\:\mathrm{cos}\:\phi\:\mathrm{cos}\:\theta+{v}\:\mathrm{sin}\:\phi\:\mathrm{sin}\:\theta={u}\:\mathrm{cos}\:\theta \\ $$$${V}−\left({u}−\lambda{V}\:\mathrm{cos}\:\theta\right)\:\mathrm{cos}\:\theta+\lambda{V}\:\mathrm{sin}^{\mathrm{2}} \:\theta={u}\:\mathrm{cos}\:\theta \\ $$$${V}−{u}\:\mathrm{cos}\:\theta+\lambda{V}\:={u}\:\mathrm{cos}\:\theta \\ $$$$\Rightarrow{V}=\frac{\mathrm{2}{u}\:\mathrm{cos}\:\theta}{\mathrm{1}+\lambda} \\ $$$$\mathrm{tan}\:\phi=\frac{{v}\:\mathrm{sin}\:\phi}{{v}\:\mathrm{cos}\:\phi}=\frac{\lambda{V}\:\:\mathrm{sin}\:\theta}{{u}−\lambda{V}\:\:\mathrm{cos}\:\theta}=\frac{\frac{\mathrm{2}\lambda{u}\:\mathrm{cos}\:\theta\:\mathrm{sin}\:\theta}{\mathrm{1}+\lambda}}{{u}−\frac{\mathrm{2}\lambda{u}\:\mathrm{cos}^{\mathrm{2}} \:\theta}{\mathrm{1}+\lambda}} \\ $$$$\Rightarrow\mathrm{tan}\:\phi=\frac{\lambda\:\mathrm{sin}\:\mathrm{2}\theta}{\mathrm{1}−\lambda\:\mathrm{cos}\:\mathrm{2}\theta} \\ $$$$\Rightarrow\phi=\mathrm{tan}^{−\mathrm{1}} \frac{\lambda\:\mathrm{sin}\:\mathrm{2}\theta}{\mathrm{1}−\lambda\:\mathrm{cos}\:\mathrm{2}\theta} \\ $$$$\Rightarrow{v}=\frac{\lambda{V}\:\mathrm{sin}\:\theta}{\mathrm{sin}\:\phi}=\frac{{u}\:\sqrt{\lambda^{\mathrm{2}} +\mathrm{1}−\mathrm{2}\lambda\:\mathrm{cos}\:\mathrm{2}\theta}\:}{\mathrm{1}+\lambda} \\ $$
Commented by ajfour last updated on 14/Sep/18
$${We}\:{both}\:{are}\:\left({hopefully}\right)\:{correct} \\ $$$${now},\:{Sir}! \\ $$
Commented by MrW3 last updated on 14/Sep/18
$${thanks}\:{for}\:{helping}\:{me}\:{to}\:{get}\:{the} \\ $$$${right}\:{solution}! \\ $$
Answered by ajfour last updated on 14/Sep/18
$$\mathrm{sin}\:\theta\:=\:\frac{{r}}{{R}+{r}}\: \\ $$$${MV}\mathrm{cos}\:\theta\:+{mv}\mathrm{cos}\:\phi\:=\:{mu}\:\:\:…\left({i}\right) \\ $$$${MV}\mathrm{sin}\:\theta\:=\:{mv}\mathrm{sin}\:\phi\:\:\:\:\:\:\:\:\:\:\:\:\:….\left({ii}\right) \\ $$$${V}−{v}\mathrm{cos}\:\left(\theta+\phi\right)=\:{u}\mathrm{cos}\:\theta\:\:\:\:….\left({iii}\right) \\ $$$${using}\:\left({ii}\right)\:{in}\:\left({i}\right) \\ $$$${MV}\mathrm{cos}\:\theta+{MV}\mathrm{sin}\:\theta\mathrm{cot}\:\phi={mu}\: \\ $$$${let}\:\frac{{m}}{{M}}={q} \\ $$$$\Rightarrow\:\:{V}\left(\mathrm{cos}\:\theta+\mathrm{sin}\:\theta\mathrm{cot}\:\phi\right)={qu}\:\:\:…\left({a}\right) \\ $$$${using}\:\left({ii}\right)\:{in}\:\left({iii}\right) \\ $$$${V}−\frac{{MV}\mathrm{sin}\:\theta\mathrm{cos}\:\left(\theta+\phi\right)}{{m}\mathrm{sin}\:\phi}\:=\:{u}\mathrm{cos}\:\theta\:\: \\ $$$${V}\left(\mathrm{1}−\frac{\mathrm{sin}\:\theta\mathrm{cos}\:\left(\theta+\phi\right)}{{q}\mathrm{sin}\:\phi}\right)={u}\mathrm{cos}\:\theta\:\:..\left({b}\right) \\ $$$${eq}.\left({b}\right)\:\boldsymbol{\div}\:{eq}.\left({a}\right)\:\:{gives} \\ $$$$\frac{\left[\mathrm{1}−\frac{\mathrm{sin}\:\theta\mathrm{cos}\:\left(\theta+\phi\right)}{{q}\mathrm{sin}\:\phi}\right]}{\mathrm{cos}\:\theta+\mathrm{sin}\:\theta\mathrm{cot}\:\phi}\:=\:\frac{\mathrm{cos}\:\theta}{{q}} \\ $$$$ \\ $$$$\Rightarrow\:{q}\mathrm{sin}\:\phi−\mathrm{sin}\:\theta\mathrm{cos}\:\theta\mathrm{cos}\:\phi \\ $$$$\:\:\:+\mathrm{sin}\:^{\mathrm{2}} \theta\mathrm{sin}\:\phi\:=\:\mathrm{cos}\:^{\mathrm{2}} \theta\mathrm{sin}\:\phi \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:+\mathrm{sin}\:\theta\mathrm{cos}\:\theta\mathrm{cos}\:\phi \\ $$$$\Rightarrow\:\left({q}−\mathrm{cos}\:^{\mathrm{2}} \theta+\mathrm{sin}\:^{\mathrm{2}} \theta\right)\mathrm{sin}\:\phi \\ $$$$\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:=\:\mathrm{2sin}\:\theta\mathrm{cos}\:\theta\mathrm{cos}\:\phi \\ $$$$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \\ $$$${or}\:\:\:\mathrm{tan}\:\phi\:=\:\frac{\mathrm{sin}\:\mathrm{2}\theta}{{q}−\mathrm{cos}\:\mathrm{2}\theta} \\ $$$$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \\ $$$${Now}\:\:\:{V}=\frac{{qu}}{\mathrm{cos}\:\theta+\mathrm{sin}\:\theta\mathrm{cot}\:\phi}\:\:\:\left[{see}\left({a}\right)\right] \\ $$$$\:\:\:\:\:\:\:\:=\:\frac{{qu}\mathrm{sin}\:\mathrm{2}\theta}{\mathrm{cos}\:\theta\mathrm{sin}\:\mathrm{2}\theta+{q}\mathrm{sin}\:\theta−\mathrm{sin}\:\theta\mathrm{cos}\:\mathrm{2}\theta} \\ $$$$ \\ $$$$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \\ $$$$\:\:\:\:\Rightarrow\:\:\boldsymbol{{V}}\:=\:\frac{\mathrm{2}{qu}\mathrm{cos}\:\theta}{\mathrm{1}+{q}}\:\: \\ $$$$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \\ $$$$\:\:\:\:\:\:\:\:\:{v}\:=\:\frac{{MV}\mathrm{sin}\:\theta}{{m}\mathrm{sin}\:\phi} \\ $$$$\:\:\:\:\:\:=\:\frac{\mathrm{2}{u}\mathrm{sin}\:\theta\mathrm{cos}\:\theta}{\mathrm{1}+{q}}×\frac{\sqrt{\mathrm{sin}\:^{\mathrm{2}} \mathrm{2}\theta+\left({q}−\mathrm{cos}\:\mathrm{2}\theta\right)^{\mathrm{2}} }}{\mathrm{sin}\:\mathrm{2}\theta} \\ $$$$ \\ $$$$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \\ $$$$\:\:\boldsymbol{{v}}\:=\:\boldsymbol{{u}}\left(\frac{\sqrt{\mathrm{1}+{q}^{\mathrm{2}} −\mathrm{2}{q}\mathrm{cos}\:\mathrm{2}\theta}}{\mathrm{1}+{q}}\:\right)\: \\ $$$$\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\:. \\ $$