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!

Inclined planes

  1. Jan 31, 2015 #1
    1. The problem statement, all variables and given/known data
    A 2 kg object is placed on a plane inclined at an angle of 30 degrees. If the coefficient of static friction is 1, and the coefficient of kinetic friction is 0.1, what is the net force on the block?

    2. Relevant equations
    F = ma

    3. The attempt at a solution
    I made a FBD with a force table of the following: Fn, w, and Ff. My acceleration along the x is -4.1 m/s2 and Fn = +17.3 N.

    My confusion is the question of the sum of the forces. If I add up all the forces along the x and the y, I get a net sum of -25.6 N. The correct answer is 2.7N. Help please!
     
  2. jcsd
  3. Jan 31, 2015 #2

    Nathanael

    User Avatar
    Homework Helper

    I don't understand how the 'correct' answer is possible. I don't agree with your answer either, so please show your work.

    Firstly, how did you get Fn=17.3?
     
  4. Jan 31, 2015 #3
    Ok. I did this problem over again slowly. I made a force table. I have normal force, weight, and force of friction. I have +17.3 for the normal force in the y direction by mg cos θ); weight is wx = -10 N and xy = -17. N. My force of friction is μFn = (0.1)(17.3) = 1.73 N.

    My net force is perpendicular to the incline = 1.73 N. What am I doing wrong here?
     
  5. Jan 31, 2015 #4

    Nathanael

    User Avatar
    Homework Helper

    Oh, I see. You're using g=10m/s/s

    Have you checked if the object will even accelerate?
     
  6. Jan 31, 2015 #5
    Yes, acceleration is -4.1 m/s2. I calculated this using Fx=max
     
  7. Jan 31, 2015 #6

    Nathanael

    User Avatar
    Homework Helper

    But you used the coefficient of kinetic friction which assumes the block does slide. You need to check if the block will even slide in the first place using static friction.
     
  8. Jan 31, 2015 #7
    Oh I see. So the key to this problem is determining the normal force? Even if I calculated the normal force, how do I determine if the object is stationary or moving?
     
  9. Jan 31, 2015 #8

    Nathanael

    User Avatar
    Homework Helper

    The normal force balances the component of gravity perpendicular to the surface. The object will be stationary if the component of gravity parallel to the surface is less than the maximum force of static friction.
     
  10. Jan 31, 2015 #9
    Ok. So if F parallel = -10 N and the Fs=17.3, then the block will be stationary? Are we comparing +/- signs here or just considering magnitude of forces?
     
  11. Jan 31, 2015 #10
    How best to approach a problem that involves both static and kinetic friction? Should I always test out the static frictional force first? For example, if the acceleration turns out to be positive (contrary to my diagram), should I assume that it is stationary? I hope that makes sense.
     
  12. Jan 31, 2015 #11

    Nathanael

    User Avatar
    Homework Helper

    Yep

    You can just compare magnitudes. The force of static friction will naturally have the opposite sign (because it opposes the force trying to make it to slide). It wouldn't make much sense for the friction to help accelerate the object, friction opposes the motion of the object.

    Well it depends on the problem. The key thing about this problem is that it says "an object is placed on incline." So you should check static friction first because the object starts out at rest. If the problem said, "someone slides an object down an incline ... " then you would not need to check the static friction because the object would already be sliding.

    You would just need to compare magnitudes. Which direction is positive and which is negative is completely arbitrary. The friction will oppose the motion of the object regardless of how you set up your coordinates.
     
  13. Jan 31, 2015 #12
    Thank you!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Inclined planes
  1. Inclined Plane (Replies: 4)

  2. Inclined planes (Replies: 5)

  3. Incline Plane (Replies: 1)

Loading...