(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

the rotor blades of a helicopter propel 2500 kg of air vertically downwards every second. The air, initially at rest, is accelerated to a speed of 15 ms^{-1}. If the helicopter starts to rise with an acceleration of 12.5 ms^{-2}, the mass of the helicopter is...

a. 1680 kg

b. 2500 kg

c. 2000 kg

d. 3000 kg

e. 3750 kg

2. Relevant equations

F = dp/dt

3. The attempt at a solution

let m_{1}= mass of helicopter and m_{2}= mass of air

When the helicopter reaches 15 ms^{-1}:

F = dp / dt

m_{1}g = d(m_{2}v) / dt

m_{1}g = v dm_{2}/dt

m_{1}= 15 * 2500 / 10 = 3750 kg

But in the manual, the answer is (a). Where is my mistake? Thanks

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# Homework Help: Newton's second law to find mass

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