JEE Main · 2023 · 11 Apr 2023 · Shift 1 · Kinematics

A projectile fired at

3 0^{\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}

)

Answer: 80

Solution

Equal heights at

3

s and

5

s are symmetric about the instant of greatest height, so the total time of flight is

T = 3 + 5 = 8

s. Since

T = \frac{2 u sin θ}{g}

8 = \frac{2 u sin 3 0 ^{\circ}}{10} = \frac{u}{10}

. Therefore the speed of projection is

u = 80

m/s.

JEE Main · 2023 · 11 Apr 2023 · Shift 1 · Integer answer

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