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A rocket of mass 4000 kg is fired vertically. What mass of gas must be ejected each second so that the rocket has an initial upward acceleration of 19.6 m/s²? (Exhaust speed of fuel = 980 m/s, g = 9.8 m/s².)

1. 240 kg s⁻¹
2. 60 kg s⁻¹
3. 120 kg s⁻¹
4. None

Correct answer: 120 kg s⁻¹

Solution

Thrust = u*(dm/dt) = m(g + a); so dm/dt = 4000*(9.8 + 19.6)/980 = 120 kg/s.

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