A projectile fired at 30^{circ} to ground is observed to be at same height at time 3 s and 5 s after

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3-2.Motion in Plane

hard

A projectile fired at $30^{\circ}$ to the ground is observed to be at same height at time $3 s$ and $5 s$ after projection, during its flight. The speed of projection of the projectile is $.........\,ms ^{-1}$(Given $g=10\,m s ^{-2}$ )

A

$70$

B

$80$

C

$60$

D

$50$

(JEE MAIN-2023)

Solution

Time of flight $t_1+t_2=3+5=8\,sec$

$T =\frac{2 u \sin 30^{\circ}}{ g }$

$8=\frac{2 u \sin \left(30^{\circ}\right)}{10}$

$u =80\,m / s$

Standard 11

Physics

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$A.$ $\theta \, = \,{45^o}$ $1.$ $\frac{{{K_h}}}{{{K_i}}} = \frac{1}{4}$

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