20) A toy rocket is launched vertically from ground level (y = 0 m), at time t =

Question

20) A toy rocket is launched vertically from ground level (y = 0 m), at time t = 0.0 s. The rocket engine provides constant upward acceleration during the burn phase. At the instant of engine burnout, the rocket has risen to 64 m and acquired a velocity of 60 m/s. The rocket continues to rise in unpowered flight, reaches maximum height, and falls back to the ground. The time interval, during which the rocket engine provides upward acceleration, is closest to: A) 1.5 s B) 2.3 s C) 1.9 s D) 1.7 s E) 2.1 s 21) A toy rocket is launched vertically from ground level (y = 0 m), at time t = 0.0 s. The rocket engine provides constant upward acceleration during the burn phase. At the instant of engine burnout, the rocket has risen to 81 in and acquired a velocity of 40 m/s. The rocket continues to rise in unpowered flight, reaches maximum height, and falls back to the ground. The upward acceleration of the rocket during the bum phase is closest to: A) 9.9 m/s^2 B 9.6 m/s^2

Explanation / Answer

S= 64m

v=60m/s

v*v = 2aS

=>a = 60*60/2*64 = 28.125m/s2

now we know t = v-u/a

=>t = 60/28.125 = 2.13

hence option E

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