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, on ontop of the other

  1. Jul 29, 2007 #1
    1. The problem statement, all variables and given/known data
    block A is ontop of block B.
    block A more mass than B

    Which force is greater magnitude, Force of gravity (mg (weight) on block B, or force of block A on B?
    Wouldnt answer have to be Faonb because both blocks are subject to gravity?

    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jul 29, 2007 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Indirectly, your answer is correct, but you need to be a bit more specific in your reasoning. If you believe the force of gravity on A is equal to the force of A on B, you must state why.
     
  4. Jul 29, 2007 #3
    Sorry, let me add a little more (important) info
    Both blocks are stationary on level ground.

    So lets say i want to know which force, on B has the greatest magnitude.
    I have 3 forces acting On block B.
    1) W (on b by earth)
    2) F (on b by a)
    3) N (on b by s)

    Objects are not accerating (or moving) so I know Fnet=0
    specifically,
    Fxnet = 0 and
    Fynet = 0

    Also, there are no forces in x direction

    For y I have
    SumFynet = NyBS - mgyBE - FyBA= 0

    So just purely from above equation, largest force would have to be the normal force of suface on B.
    But how do i know which force is bigger, force on B by Earth (FyBE), or force on B by A (FyBA)??
    thanks for any help
     
  5. Jul 30, 2007 #4

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Good FBD of block B. But now to answer your question, you need to make a FBD of block A, then use Newton 3.
     
  6. Jul 30, 2007 #5
    mujadeo, you clearly understand the process, so just finish your thought. The weight of B is the force from gravity. Since A is on top of B, and nothing is moving, the weight of A, would be the force acting on B. You know that A has greater mass than B, gravity is the same for both blocks, so which force is greater?
     
  7. Jul 30, 2007 #6
    "You know that A has greater mass than B, gravity is the same for both blocks, so which force is greater?"
    yeah thats how i was visualizing it, so thanks for the confirmation.
    Obviously A would have the greater Force (gravity times its mass, (which is greater than B))

    One other querstion though: Necause system is stationary (Fnet=0) that just means that all the forces must add to zero, right?
    OK so does that mean when i do FBD for block A, that should = 0
    and FBD for B that should be 0
    and FBD for system A and B, that should 0 also, is that rght? ---thanks
     
  8. Jul 30, 2007 #7

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes to all your questions.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook