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Homework Help: Non inertial pendulum problem

  1. Oct 29, 2006 #1

    I have this practice problem i'm trying to figure out.

    A pendulum 80 cm in length has a mass of 0.4g is suspended on the roof of a truck accelerating at 2.6m/s/s. Find a) the horizontal deviation of the mass; b) the tension in the cord.

    For a, I know the answer is 20.6cm but I really have no clue as to where to start, what formulas to use. I've been trying for a couple of hours with the formulas I have and can't seem to get the correct answer. I do know this would be a non-inertial frame of reference type of problem but i'm stuck as to how I arrive to that answer.

    For b, I thought i'd use the pythagorean equation since we have the weight and the force F' making a 90 degree angle. So I did sqrt((mg)^2 + (ma)^2) = 4.06N.

    Any insight on how I should approach the first problem and the steps I should take to solve it would be greatly appreciated!

  2. jcsd
  3. Oct 29, 2006 #2


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    For (a) start by drawing a FBD of the mass. Note that the mass will be accelerating at the same rate as the truck.
  4. Oct 29, 2006 #3
    Ok. So there are 3 forces (mg, ma and T) acting on the object.

    Fx = Tcos(theta) - F'
    Fy = Tsin(theta) - mg

    Is that right? If so, where do I go from here?
  5. Oct 29, 2006 #4
    Carefull, if you are saying that theta is the angle with the verticle, then you mixed up the sine and cosine.
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