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!

Normal force and friction

  1. Feb 16, 2017 #1
    • Member advised to use the homework template for posts in the homework sections of PF.
    Hi all,
    I have problem to solve this question:

    2iiwydi.jpg
    Wb=150N
    Wa=50N
    T=150N
    alph =?
    µ =?
    - There is friction between A and B only(Between A and the ramp there isn't).
    - A and B not moving(first Newton's law).

    3. I tried to solve as:
    kcx8gk.jpg

    But the answer is wrong, can anyone help me please, and explain where is the problem?

    Regards Jack.
     
  2. jcsd
  3. Feb 16, 2017 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    What is your answer?
     
  4. Feb 16, 2017 #3
    The correct anser is Miu=0.25
    My answer is 0.43.
     
  5. Feb 16, 2017 #4

    ehild

    User Avatar
    Homework Helper
    Gold Member

    How did you get it?
     
  6. Feb 16, 2017 #5
    By writing the forces as i the diagram forces, did the diagram, i don't care about the final answer and don't care about solving and finding the Miu, what i want to know is if the forces are right as the diagram or not, if not what is the problem, in other world i want to understand the physics not from the question from who understand physics and can explain.
     
  7. Feb 16, 2017 #6
    Hi Physics86. Welcome to Physics forums. It looks like your diagrams are correct. The N??? in diagram for A would be equal to Nb because those forces have to be equal and opposite. I worked through the problem and got μ = 0.25.
     
  8. Feb 16, 2017 #7

    ehild

    User Avatar
    Homework Helper
    Gold Member

    You need to collect the forces acting on block A. You have some confusion here, as there is normal force from the ramp (Na) and there is normal force from B acting on A, that can be the blue vector N???.
    The forces acting on B are correct, taking in mind that the normal force Nb is the force exerted by A, and according to Newton's Third Law, it is equal in magnitude to N???.
    You need to use the correct magnitudes to get the angle alpha. Remember, the friction is static, so F≤μN, and N is the magnitude of the normal force between A and B.
     
  9. Feb 16, 2017 #8
    F≤μN. That is an important point. I should have pointed out that, although it wasn't stated, I assumed that one of the conditions of the problem was that block A was just at the point where it was going to slip. In that case, F would be equal to μN (F=μN). Otherwise - if that was not a condition of the problem - you would not be able to calculate μ.
     
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: Normal force and friction
Loading...