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!

Homework Help: 2 blocks, frictions, and no numbers

  1. Sep 28, 2008 #1
    Block A, with mass mA, is initially at rest on a horizontal floor. Block B, with mass mB, is initially at rest on the horizontal top surface of A. The coefficient of static friction between the two blocks is μs. Block A is pulled with a horizontal force. It begins to slide out from under B if the force is greater than:

    A. mAg
    B. mBg
    C. μsmAg
    D. μsmBg
    E. μs(mA+mB)g

    I came up with a couple of equations (that may not be right).

    f = μmBg
    f = F - mAaA

    So I tried substitution and solving for F, but I can't get the right answer (E).
    So what's the right way to do this problem?
  2. jcsd
  3. Sep 28, 2008 #2
    The equation for frictional force is the coefficient of friction times the normal force. What is the normal force on block A?
  4. Sep 28, 2008 #3
    It would be mBg, I guess.
  5. Sep 28, 2008 #4
    Really? Draw a free body diagram.

    Don't forget that block B is sitting on top of block A.
  6. Sep 28, 2008 #5
    I did.

    I have force F acting on mA.
    mAg is directed down
    mBg is directed down, and then up as a normal force
    friction is in opposite direction of the 2 blocks

    What am I doing wrong here??? =(
  7. Sep 28, 2008 #6
    The problem is that block A has block B on top of it. That means that the weight of block A is (Ma + Mb)g. What that means is that since block A is not accelerating down, it's normal force must be the same as the weight. Now do you understand?
  8. Sep 28, 2008 #7
    Ok, I see now. So E says that F = μNA
    But what does that mean? I thought friction is supposed to be the same for both blocks in this problem. This is getting more confusing.
  9. Sep 28, 2008 #8
    The coefficient of friction is the same. The frictional force depends upon the normal force though. Because A has a greater normal force than B, it would produce a greater frictional force.
  10. Sep 28, 2008 #9
    I see. Thanks for the explanations. Now I really need to study hard for that test.
  11. Sep 28, 2008 #10
    No problem :) Glad I could help. Good luck!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook