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Homework Help: Discrepancy with Normal Force Problems

  1. Jul 8, 2015 #1
    As I try to set up equations regarding Newtonian forces, I am looking for patterns in example problems I read. It's really frustrating then when I see different approaches that I can't see a connection to.

    There are two problems that I've attached that seem to solve - what seems to me about the same content - differently.

    First, the "physics book" file. This is a problem from on Forces and Newton's Laws of Motion. When calculating Fx, this problem does NOT take into account FN (Normal force). Other problems I have seen set up like this say the "x component of FN is 0, and the y component of FN is FN. The x components in this problem are dealing with W and T, which I get.

    Next, look at the screen shots (in succession) I took of my online instructional of the same topic. Here you will see that for a similar object in a similar situation (a block that is on a ramp), they say the x component of FN is FN*cos90, and the y component is FN*sin90. It seems strange because since they have it lined up on the y axis, would not the x component be 0, and the y component be FN? I don't get the rationale for accounting for the cos/sin90 part. I know we are trying to find the angle ultimately, but why the difference in the approaches and how do I differentiate between the two?
    Thank you.

    Attached Files:

  2. jcsd
  3. Jul 8, 2015 #2


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    And what is the value of cos 90?
  4. Jul 8, 2015 #3
    Oh boy - I feel silly. Zero, of course. Thank you Haruspex. I was so focused on the set up of the problem I looked past that. Then the sin90*FN would simply be FN, as I originally thought. And I also tried to do the screen shot problem in a slightly different way (I tried the ramp sloping down from the right, whereas they had it down from the left), thinking I would get the same answer, but I did not. So I need to go back and see what I did wrong. Anyway, thank you again for this simple insight!!
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