Q1)A 5000 kg rocket is set for firing.The exhaust speed is 800m/s. To

1.	Q1)A 5000 kg rocket is set for firing.The exhaust speed is 800m/s. To givean initial upward acceleration of 20m/s2, the amount of gas ejected persecond to supply the needed thrust willbe [AFMC1997]a) 137.5 kg/secb) 185.5 kg/secc) 125 kg/secd) 187.5kg/sec
Answer» In given case mass of rocket is changing with time, but velocity of gases remained constant.Therefore Thrust=v (dm/dt) -eq(1)Where dm/dt is rate at which mass of gas is ejected.Resultant acceleration = Acceleration due to thrust (up) - acceleration due to gravity(down) 20 = a - 10 Or a = 30 is the acceleration produced by thrust According newton's second law of thrust=(mass)×(acceleration)...(2) From equation (1) and (2) we getma=v ( dm/dt) 5000 × 30=800 ( dm/dt ) On solving equation for (dm/dt)we get(dm/dt)=187.5 kg/sec Answer:(c)

Q1)A 5000 kg rocket is set for firing.The exhaust speed is 800m/s. To givean initial upward acceleration of 20m/s2, the amount of gas ejected persecond to supply the needed thrust willbe [AFMC1997]a) 137.5 kg/secb) 185.5 kg/secc) 125 kg/secd) 187.5kg/sec

Answer»

In given case mass of rocket is changing with time, but velocity of gases remained constant.Therefore Thrust=v (dm/dt) -eq(1)Where dm/dt is rate at which mass of gas is ejected.Resultant acceleration = Acceleration due to thrust (up) - acceleration due to gravity(down) 20 = a - 10 Or a = 30 is the acceleration produced by thrust According newton's second law of thrust=(mass)×(acceleration)...(2)

From equation (1) and (2) we getma=v ( dm/dt) 5000 × 30=800 ( dm/dt ) On solving equation for (dm/dt)we get(dm/dt)=187.5 kg/sec Answer:(c)

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