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Homework Help: Newton's 3rd law help

  1. Feb 6, 2008 #1
    Could someone please check my work and verify if I am correct? I am not confident about any of my answers except #1. Thank you very much for your time.

    1. The initial mass of a giant rocket is 25000kg. What is the weight of the rocket?
    Answer: 250,000 N

    2. Assume 90% of the mass of the rocket is fuel and is spewed out nearly instantaneously. If the fuel for the rocket travels downward at a velocity of 50 m/s after being accelerated for 2 seconds then what is the force on the rocket's fuel? Ignore gravity on this one.
    Answer: A=change in velocity/T so a=25 m/s^2. Then I did F=MA so F=22,500kg*25m/s = 562,500 Is this correct?

    3. A.What is the net force on the rocket (remember gravity)?
    Answer: F=MA so 2500kg*10m/s= 25,000N I am really iffy about this one

    B. What is the acceleration of the rocket?
    Answer: A=f/m so a=25,000N/2500kg= 10m/s^2 ????????

    C. After 2 seconds how fast will the rocket be traveling?
    Answer: V=at so V=10m/s^2 * 2s= 20m/s ???

    4. Somehow we get the rocket into orbit around the earth.
    A. What is the weight of the rocket in orbit (mass = 2500 kg now)?
    Answer: F=MA so F=2500kg*10m/s= 25,000N. I wasn't sure what the acceleration would be for this one.

    B. If an astronaut applies a force of 750N to the rocket then what will the acceleration of the rocket be?
    Answer: A=F/M so A=25,750N??/2500 kg = 10.3 m/s^2 ?????

    C. The astronaut forgot to tether himself! What will the acceleration of the astronaut be (mass of astronaut is 100N)?
    Answer: A=F/M so A= 750N/100N= 7.5 m/s^2

    Again thanks for your time. I really need some help with this
  2. jcsd
  3. Feb 6, 2008 #2
    All correct.
  4. Feb 6, 2008 #3
    Thanks for the quick response.

    Anyone else care to verify that I am correct?
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