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Help with Force and Acceleration

  1. May 29, 2007 #1
    Problem 1:
    An accelerometer - a device to measure acceleration - can be as simple as a small pendulum hanging in the cockpit. Suppose you are flying a small plane in a straight horizontal line and your accelerometer hangs α = 14.4° behind the vertical, as shown in the figure below.
    [​IMG]

    What is your acceleration at that time?
    in the direction of motion

    I figure you use F=MA for this, but there is no mass given... i dont even know where to begin, seeing that the only thing given is the angle.


    Problem 2:

    A fire helicopter carries a 561 kg bucket of water at the end of a 24.9 m long cable. Flying back from a fire at a constant speed of 43.6 m/s, the cable makes an angle of 48.4° with respect to the vertical. Determine the force of air resistance on the bucket.

    This is like the last problem, i was able to find all the sides of the triangle which is made by the weight, but dont know what to do now.

    And Problem 3:
    A 2016-kg elevator moves with an upward acceleration of 1.86 m/s2. What is the tension in the cable that supports the elevator?
    I used F=MA but it said the answer was wrong.

    Thanks for the help.
     
  2. jcsd
  3. May 29, 2007 #2
    Well for problem three the tension on the cable is the weight plus the force required to accelerate the elevator at 1.86m/s2.
     
  4. May 29, 2007 #3

    Doc Al

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    Staff: Mentor

    Problem 1

    Yes, Newton's 2nd law is what you need here (and in all three problems). Start by identifying the forces acting on the pendulum bob and then apply Newton's law. (You don't need to know the mass.)
     
  5. May 29, 2007 #4
    so basically i do: (MA)+(2016*9.8)

    The forces are velocity and air resistance? im not really sure where to go from there?
     
  6. May 29, 2007 #5

    Doc Al

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    Staff: Mentor

    Velocity is not a force; there's no air resistance in this problem.

    Hint: Two forces act on the pendulum mass. What are they?
     
  7. May 29, 2007 #6
    motion and gravity?
     
  8. May 29, 2007 #7

    Doc Al

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    Staff: Mentor

    "Motion" is not a force. But gravity is. For the other force, consider the tension in the string that pulls on the mass.

    Now consider the vertical and horizontal components of these forces. Apply Newton's 2nd law to each direction.
     
  9. May 29, 2007 #8
    ok so there is the Fg (force of gravity) which points straight down from the mass and Fn (normal force) points straight up. Then i labeled Fn as being 9.8 and having an angle of 75.6 deg. i set it up as being: sin(75.6)x = 9.8 where x= the string from the top to the mass. Is this right? I got 10.12 m/s^2 and it says that it is wrong.
     
    Last edited: May 29, 2007
  10. May 29, 2007 #9
    i got it. 2.52 m/s^2!!!! now what do i do for the other 2? im still stuck.
     
  11. May 30, 2007 #10

    Doc Al

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    Staff: Mentor

    Problem 2

    Again, identify all the forces acting on the object--the bucket in this case. Then apply Newton's 2nd law to the horizontal and vertical force components.

    Hint: Three forces act on the bucket.
     
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