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Homework Help: Force - Scale - Tension

  1. Oct 6, 2007 #1
    On takeoff, the combined action of the engines and wings of an airplane exerts an 8000N force on the plane, directed upward at an angle of 65* above the horizontal. The plane rises with constant velocity in the vertical direction while continuing to accelerate in the horizontal direction.
    a) What is the weight of the plane ?
    b) What is its horizontal acceleration ?
    V (initial)= 0
    weight = mass time gravity
    weight = 9.8 time mass
    acceleration in vertical direction = 0 (constant speed)
    F (apply) = 8000 N
    From F = ma --> m = F / a
    And I stuck at this point !

    Two blocks are fastened to the ceiling of an elevator. Each block is 10 kg. The elevator accelerates upward at 2 m/s2 (at the block that close to the ceiling). Find the tension in each rope.

    F (upper) = 100 N
    F (lower) = 100 * 2 = 200 N

    A 72 kg man stands on a spring scale in an elevator. Starting from rest, the elevator ascends, attaining its maximum speed of 1.2 m/s in .80s. It travels with this constant speed form 5 s, undergoes a uniform negative acceleration for 1.5 s and comes to rest. What does the spring scale registered (a) before the elevator starts to move ? (b) during the first .8 s ? (c) while the elevator is traveling at constant speed ? (d) during the negative acceleration ?
    a) Weight = 72*9.8 = 705.6 N because Fg = m*g
    b) From V (intial) = 0 V (final) = 1.2 m/s t = .8 s
    so a = 1.5 m/s2.
    F = 705.6 + F (that is accelerating) ==> 705.6 + 72 (1.5) = 813.6 N
    c) 705.6 N
    d) I don't get what is this say "undergoes a uniform negative acceleration for 1.5 s and comes to rest"

    Help Please ? Thanks
  2. jcsd
  3. Oct 7, 2007 #2


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    1. How much force is airplane exerting vertically
    2. How much force is it exerting horizontally
    I'm not sure how you've calculated 100N, but it's wrong.
    Answers (a) through (c) are correct. (d) simply means decceleration.
  4. Oct 7, 2007 #3
    Can you give me some hints for problem 2 please ?
  5. Jan 9, 2010 #4
    Sorry to bring up an old dead topic but I just got problem 3 for homework. What I did for d:
    Find acceleration: a=(0Vf-1.2Vo(top speed of elevator ascending))/1.5time stated = -.8.
    Net force = 705.6Normal weight+((72mass)x(-.8acceleration)) = 648N

    Any mistakes?
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