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Homework Help: Circular Motion question. Need direction

  1. Oct 16, 2005 #1
    Hey all, I'm new here. I like physics, but I have a hard time with it. It's frustrating because I have a problem of not knowing what all is going on in some problems. The physics tutor at my school is hard to get ahold of so I decided to look online for physics related items. I stumbled across this site and figured it would be a good place to refine my skills.

    Well here is my question.

    A train traveling at a constant speed rounds a curve of radius 235m. A lamp suspended from the ceiling swings out to an angle of 17.5 throughout the curve. What is the speed of the train?

    I see that the speed of of the lamp=speed of the train. Since an angle is involved I initially thought it would be a banking problem, but without a weight to work with I decided that is probably didn't have to do with bankment.

    Since the speed is constant does that mean acceleration is also constant in this case?
    I know speed and velocity aren't the same in some cases, but I think the speed and velocity are the same in this situation.

    I looked at the lamp as being at 270 degree's initially, and 17.5 was a reference angle (or maybe the 197.5 is the reference angle :p. I forget). So I added 17.5 and 180 to get 197.5 degree's if you were looking at it as a circle. I'm uncertain where to go from here.

    Any help would be appreciated. Thanks
    Last edited: Oct 16, 2005
  2. jcsd
  3. Oct 16, 2005 #2
    This isnt correct. The lamp is suspended, meaning it isnt moving relative to the train. The key point here is that the lamp is being pulled forward by the train and pulled back by the tension in the string. You should notice that the net force on the lamp is 0, thats the most important part here.

    This is a case where speed and velocity are NOT equal. Speed is the MAGNITUDE of the velocity vector. The velocity vector here changes direction as you travel around the curve, but the speed remains the same.

    The way to tackle this is to use the angle of the lamp's displacement to find the speed. Draw a force diagram for the lamp, and set up equations for the tension in the string in the vertical and horizontal direction.

    You only need to work in two dimensions here, don't confuse yourself and make it a three dimension problem.
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