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Homework Help: Simple kinematics question?

  1. Jul 17, 2011 #1
    1. The problem statement, all variables and given/known data
    A metal box, attached to a small parachute, is dropped from a helicopter.

    The total force opposing the motion of the box and parachute at a particular instant during its fall is 30N. The combined mass of the box and parachute is 5.0 kg. Calculate the resultant downward force on the box and parachute (assume g = 10 m/s²).

    At the end of the fall, the parachute is caught on a tall tree. The box is then cut loose and falls from rest to the ground. The time of fall is 2.4 seconds. Calculate

    i) the velocity which the box strikes the ground.
    ii) average velocity during the fall
    iii) the distance fallen

    2. Relevant equations

    3. The attempt at a solution

    I calculated the resultant force of the box and parachute to simply be:

    F(resultant) -f = ma
    F(resultant) - 30 ( 5 x 10)
    F(resultant) = 20N

    I have also calculated the velocity which the box strikes the ground.

    i) a = (v-u)/t
    10 = v-0/2.4
    v =24m/s

    For the average velocity during the fall, can someone advise me what would it be?
  2. jcsd
  3. Jul 17, 2011 #2

    Doc Al

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    Staff: Mentor

    Correct answer, but be careful with your reasoning. This is not an 'F=ma' problem, but just a problem in adding vectors to find ΣF.


    You have the initial and final velocities, so what do you think?
  4. Jul 17, 2011 #3
    Is it simply (24+0)/2 ? Correct me if I am wrong please.
  5. Jul 17, 2011 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Yes, that is correct.
  6. Jul 17, 2011 #5
    Thank you very much.
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