Rocket/Variable mass

  • Thread starter hjr
  • Start date
  • #1
hjr
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Homework Statement


A rocket traveling 1950 m/s away from the Earth at an altitude of 6600 km fires its rockets, which eject gas at a speed of 1300 m/s (relative to the rocket).
If the mass of the rocket at this moment is 2.25×104 kg and an acceleration of 1.4 m/s^2 is desired, at what rate must the gases be ejected?

Homework Equations



M*dv/dt = [tex]\sum[/tex] F(ext) + v(rel)*dM/dt

The Attempt at a Solution



I thought i could just solve for dM/dt in the above equation and plug in the numbers to get the answer but that didnt work. I think my problem is with finding all the external forces, which i think is just mg. I just need a hint to start.
 

Answers and Replies

  • #2
hjr
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*bump*
 
  • #3
ehild
Homework Helper
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The external force at altitude of 6600 km is not mg.

ehild
 
  • #4
hjr
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The external force at altitude of 6600 km is not mg.

ehild

so when I find g is that all i have to find? ->for the external force?
 
Last edited:
  • #5
ehild
Homework Helper
15,543
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The external force is gravity that depends on the altitude.
F=GmM/r2, and r is the distance from the centre of Earth, r=radius or Earth + altitude

ehild
 
  • #6
hjr
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alright thanks
 

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