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Homework Help: Newton laws

  1. Mar 21, 2010 #1
    1. The problem statement, all variables and given/known data

    A landing craft of mass 12,000 kg prepares itself for a Moon landing. When at a
    vertical distance 160m above the Moon’s surface it’s downwards velocity is 15 m/s.
    A retrorocket is fired to give the craft an upward thrust to slow its speed to zero as it
    meets the surface. Calculate the needed thrust to decelerate the craft its initial
    downwards velocity to zero as it lands. The value for gravity on the Moon is g/6.
    (Where g is 9.81m/s2).

    2. Relevant equations


    3. The attempt at a solution
    Gravity =9.81/6

    F (craft)= ma
    =12 000. 1.635
    Therefore the craft will need a a thrust of 19620 N upwards. Is that right or is that other information needed??
  2. jcsd
  3. Mar 21, 2010 #2
    No that is not right. That upward thrust of 19620N merely counteracts the gravitational force acting on the landing craft. As such, there will be no net force on the craft, and it will continue moving downwards at a constant velocity of 15m/s.

    In order to decelerate the craft, you are going to need a net upwards force acting on the craft. Based on the information in the question, what is the magnitude of the upward acceleration that the craft must undergo?
  4. Mar 24, 2010 #3
    ok...but i cant calculate an upward force without, Acceleration im only given constant velocity?? which means there is no acceleration
  5. Mar 24, 2010 #4


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    Homework Helper

    There has to be an acceleration for the craft's velocity to reduce to 0 m/s from 15 m/s. I think you should start by calculating the acceleration required to bring the craft's velocity down by 15 m/s in the given distance.
  6. Mar 24, 2010 #5
    ok i did that and got -0.703125 as acceleration

    then i got a force of -8437.5 N

    so the craft needs a downward force of 8437.5N to come to 0 m/s??? is it downward or upward??
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