1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Statics/physics homework

  1. Sep 11, 2011 #1
    1. The problem statement, all variables and given/known data

    problem 2/120 is the question I'm having a hard time with. I know how to do everything except I just can't EYE this one.

    I would like to solve for Mz by taking the cross product rxF. but I don't know how to get those vectors for this problem.
  2. jcsd
  3. Sep 11, 2011 #2


    User Avatar
    Homework Helper
    Gold Member

    This problem is most easily done if you express each vector in unit vector notation, then take the cross product.

    Can you write vector r in unit vector notation?

    For vector P you first need to write it in terms two unit vectors, one along the normal n and one along the z-axis. Once you have done this, write n in terms of unit vectors along the x and y axes and you're done.
  4. Sep 11, 2011 #3
    doesn't having them both in unit vector notation means you'll get a diff magntitude when taking the cross product than if you were to take the cross product without the unit vector?
  5. Sep 11, 2011 #4


    User Avatar
    Homework Helper
    Gold Member

    Perhaps you don't understand unit vector notation. If vector A has components Ax= 3 units and Ay= 4 units, we would write it in unit vector notation as
    [itex]\vec{A}=3\widehat{x}+4\widehat{y}[/itex] units,
    where [itex]\hat{x}[/itex] stands for "in the x-direction" and [itex]\hat{y}[/itex] stands for "in the y-direction". So the above equation in plain English translates as "Vector A is the same as going three units in the x-direction and then going 4 units in the y-direction." Note that the magnitude of vector A is not one but five units. You get 5 by squaring whatever multiplies i-hat, adding to it the square of whatever multiplies j-hat and then taking the square root of this sum (Pythagorean theorem.)
  6. Sep 11, 2011 #5

    Of course not.
  7. Sep 11, 2011 #6
    I'm confused now. I always thought unit vector notation was that the vector has a magnitude of 1. so for your vector wouldn't unit vector notation be 3/5i+4/5j??
  8. Sep 11, 2011 #7


    User Avatar
    Homework Helper
    Gold Member

    It would. I note that (3/5)i+(4/5)j is a unit vector (a vector of magnitude 1) that points along the direction of A. Observe that vector A, as I have written it in unit vector notation, is the magnitude of A times a unit vector in the direction of A, i.e. A=5[(3/5)i+(4/5)j] = 3i+4j units. In this problem, for r, you have to write down a vector that has magnitude 900 mm and looks like r = (so many mm)i+(so many other mm)j.
  9. Sep 15, 2011 #8
    I got the right answer (208k) by shifting the x&y axis by 20 degrees thus making the force perpendicular with the y axis. Is that an accurate way to do it?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Statics/physics homework
  1. Physics homework (Replies: 1)

  2. Physics Homework (Replies: 1)

  3. Physics homework. (Replies: 1)