91影视

It is given that,

$$\begin{gathered} v_1=85\mathrm{km}/\mathrm{h} \\ {\mathrm{v}}_2=650\mathrm{km}/\mathrm{h} \end{gathered}$$

Speed reduction of 20%

This problem deals with the relative velocity. Relative velocity is the velocity of an object in the rest frame of another object. To find the direction of bullet fired use the relative velocity of the bullet.

Velocity of box,

${\stackrel{鈬赌}{v}}_{cg}=v_1\hat{i}$

Bullet has velocity relative to ground is given as below,

${\stackrel{鈬赌}{V}}_{0bg}=V_{2}cs胃\hat{i}+v_2\sin 胃\hat{j}$

Velocity of bullet in car,

${\stackrel{鈬赌}{v}}_{bc}=v_3\hat{\mathrm{j}}$

As speed reduced to 20%, so new velocity is,

$$\begin{gathered} {\stackrel{鈬赌}{v}}_{bg}=0.8v_2\cos 胃\hat{\mathrm{i}}+0.8{\mathrm{v}}_2\sin 胃\hat{\mathrm{j}} \\ 0.8{\mathrm{v}}_2\cos 胃\hat{i}+0.8v_2\sin 胃\hat{\mathrm{j}}=v_3\hat{\mathrm{j}}+v_1\hat{\mathrm{i}} \end{gathered}$$

To find the direction, equate x components,

$$\begin{gathered} 0.8{\mathrm{v}}_2\cos 胃=v_1 \\ \cos 胃=\frac{v_1}{0.8{\mathrm{v}}_2} \\ \mathrm{So}, \\ \cos 胃=\frac{85\mathrm{km}/\mathrm{h}}{0.8脳650\mathrm{m}/\mathrm{s}}脳\frac{1000\mathrm{m}/\mathrm{km}}{3600\mathrm{s}/\mathrm{h}} \\ \mathrm{So}胃=87掳 \\ \mathrm{So},87掳\mathrm{is}\mathrm{the}\mathrm{direction}\mathrm{of}{\stackrel{鈬赌}{\mathrm{v}}}_{\mathrm{bg}} \end{gathered}$$

Since, the direction in which bullet was fired had been found, it would give us the answer as,$78=93掳$.