Question

a ball thrown upward with speed 10m/s from the top of the tower reaches the ground with a speed 20m/s. the height of the tower is? g=10m/s^2

Answer 1

let initial velocity u =0

so final velocity =20m/s

g=10

height of tower h =?

acc. to 3rd equation of motion

v²=u²+2gh

⇒v²-u²=2gh

⇒20²-0²=2*10*h

⇒400=20h

⇒h=400/20=20m

∴the height of the tower is 20m∴

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Answer 2

${v}^{2} - {u}^{2} = 2as \\ here \: v = 20m {s}^{ - 1} \\ u = 10m {s}^{ - 1 } \\ a = \frac{v - u}{t} \\ substitute \\ {v}^{2} - {u}^{2} = 2as \\ 400 - 100 = 2 \times 10 \times s \\ 300 = 20 \times s \\ s = 15$