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A person sitting in the ground floor of a building notices through the window, of height 1.5 m, a ball dropped from the roof of the building crosses the window in 0.1 s. What is the velocity of the ball when it is at the topmost point of the window ? (g–10 m/s2)

(1) 20 m/s

(2) 15.5 m/s

(3) 14.5 m/s

(4) 4.5 m/s

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h=ut+1/2*at^2
-1.5=u(0.1)+1/2*(-10)(0.1)^2
-15/10=u(0.1)+1/2*(-10)(1/100)
-15=10[u(0.1)+1/2*(-10)(1/100)
-15=u-0.5
u=-15+0.5
u=-14.5m/s

1 Answer

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Best answer

S= ut + 1/2 at2

-1.5=0.1u-1/2 *10*(0.1)2

u=-15.5

therefore, negative sign indicates that it is in downward motion.

Thanks guys!!

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