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Ap physics c

  1. Sep 12, 2015 #1
    1. The problem statement, all variables and given/known data
    A 15000-kg helicopter is lowering a 4000-kg truck by a cable of fixed length. The truck, helicopter, and cable are descending at 15.0 m/s and must be slowed to 5.00 m/s in the next 50.0 m of descent to prevent damaging the truck. Assume a constant rate of slowing.

    Determine the tension in the cable.

    Determine the lift force of the helicopter blades

    2. Relevant equations
    F=ma

    3. The attempt at a solution
    dont have answers but attempt was to isolate the system of truck
    MG-FT=ma
    4000*9.8-ft=4000*-2
    ft=48000?

    force of lift
    LF-mg+mg =ma
    LF-15000(9.8)+4000(9.8)=15000+4000(-2)
    LF=152000
     
  2. jcsd
  3. Sep 12, 2015 #2
    You isolated the truck but you didn't isolate the helicopter?

    Also your signs are off.
     
    Last edited: Sep 12, 2015
  4. Sep 12, 2015 #3
    why do i need to isolate the helicopter to find the lift i used the entire system
     
  5. Sep 12, 2015 #4
    Yes, you're absolutely right. Just fix your signs then.
     
  6. Sep 12, 2015 #5
    ohh u mean my 9.8 should be negative lol my bad
     
  7. Sep 12, 2015 #6
    No, you used +ve for the truck and -ve for the helicopter.
     
  8. Sep 12, 2015 #7
    You have your gravity vector pointing in two different directions.

    Also, the tension in the cable is the same everywhere, draw a FBD and cut the problem into two halves. In the top half only consider the helicopter, in the bottom half only consider the truck.
     
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