1. Not finding help here? Sign up for a free 30min 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!

Im ready for this test except I cant get started on these 2 questions.

  1. Apr 29, 2009 #1
    1. The problem statement, all variables and given/known data

    A force of 20N,180degrees (W) and 30N,-110degrees (S20degrees (W) ) are both acting on a 3.0kg mass at rest. If there is 10N of friction:calculate the acceleration.

    2. Relevant equations

    I think you just need kinematic equations so here they are:
    V2=V1+a*t
    d=V1t+1/2a*t^2

    3. The attempt at a solution
    Im going to try the R theta and change that to x and y I just wanted to put this up first stop me if this is wrong.
     
  2. jcsd
  3. Apr 29, 2009 #2
    wait you need forces equations:
    Fn=m*a
    Fn=Fa+Ff
    Fg=m*g
    Ff=m*F perpendicular sign
     
  4. Apr 29, 2009 #3

    Doc Al

    User Avatar

    Staff: Mentor

    What's the total applied force?
    Which way does the friction act?
    First find the net force acting on the mass.
     
  5. Apr 29, 2009 #4
    well the friction would be negative right since its going against the object
     
  6. Apr 29, 2009 #5
    oh wait the total applied force are the first numbers
     
  7. Apr 29, 2009 #6
    so I use Fn=Fa+Ff how do I change the first numbers into something I can use
     
  8. Apr 29, 2009 #7

    Doc Al

    User Avatar

    Staff: Mentor

    To add the two applied forces, first break them into their components. Then you can add the components and find the new resultant.
     
  9. Apr 29, 2009 #8
    im not sure how you chnge them dont you just use pol on the calculator and then F to get them but what about the S20degrees (W)
     
  10. Apr 29, 2009 #9

    minger

    User Avatar
    Science Advisor

    This problem I think is a little easier than you're making it. You started on the correct approach. Break each force into it's components. Add the two forces together to get the resultant components. From these forces, calculate the resultant force vector.

    You now have a force mangitude and direction. Focus for a moment on the magnitude. You're given the friction force. This force acts opposite of motion, so you don't need to worry about direction. Simply subtract the friction force from the total resultant force.

    You now have a resultant force and direction; divide by mass to get acceleration. I get 10.45 m/s² as the magnitude of the acceleration. Remember acceleration is a vector, you'll need to specify the direction as well.
     
  11. Apr 29, 2009 #10
    You shouldn't really need a calculator to break a vector into its components. Just trigonometry. Well, I guess you'll need the calculator to solve for the sin and cos of the angles...

    Breaking up a vector into its components should be second nature by this point in the semester/school year.
     
  12. Apr 29, 2009 #11
    using that pol F stuff makes it 10 times easier
     
  13. Apr 29, 2009 #12
    im just going to hope this isnt on the test lol I actually understand most of this unit now so I should be ok on the test
     
  14. Apr 29, 2009 #13

    minger

    User Avatar
    Science Advisor

    First of all, you need to take heed of what Jack said. Breaking vectors into components needs to be second nature by now. You can't always rely on calculator to add things in polar coordinates, hell you can't always use a calculator.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Im ready for this test except I cant get started on these 2 questions.
  1. I cant get started (Replies: 1)

Loading...