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Homework Help: Force required to negate vertical acceleration with friction

  1. Feb 19, 2016 #1
    1. The problem statement, all variables and given/known data
    Two blocks m (16kg) and M (88kg) are as shown in the figure. If the co-efficient of friction b/w the blocks is 0.38 but the surface beneath the block is frictionless, what is the minimum force required to hold m against M?

    2. Relevant equations
    Fnet = ma
    Fs = μs*Fa

    3. The attempt at a solution
    By using this equation for m Fnety = mg - Fs = 0
    One can find the force required Freq = (16*9.8) / μs to be 412.63N

    From there I have read that because that is the force required on m on M, it is not the required applied force on the system. That comes out to be around ~487N using proportions. How is this part solved formally? Please be as formal as possible because I want to get this concept down fundamentally.
    Thank you very much.

    (side question regarding the concept of force transferring through a system of mass like this one: Say there was a basketball in space weighing 5kg. If you applied a force of 10N on it alone, it would accelerate at 2m/s2. If the basketball was connected, by a nearly massless rigid rod that extended millions of miles, to a rock weighing 5000kg, and you pushed on the basketball with 10N, would its acceleration ever change or would it be 10N/5005kg from beginning to end?)
  2. jcsd
  3. Feb 19, 2016 #2


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    It's generally recommended for problems such as this, to sketch an FBD (free body diagram) for each mass (object) involved.

    What you have labeled as Fnet y is actually the minimum normal force required, between m and M, so that m doesn't slide down.
  4. Feb 19, 2016 #3
    Well it's also the net force on m in the y-direction is it not? I thought the only forces acting on m are gravity and the static force of friction through the applied force.
    FBD I drew quickly. That is what I see visually along with the image. Fa is equal in both FBD's, correct?
  5. Feb 19, 2016 #4


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    You mean, what is labelled Freq is the minimum normal force, yes?
    Obliv, what is the relationship between the normal force and F? What is the relationship between the normal force and M?
  6. Feb 20, 2016 #5
    What you have missed is the contact force effect on mass M, When you push something it moves. You have to add to that force the required force to make the two objects get the same acceleration.

    A great place to understand fcontact is http://www.physicsclassroom.com/class/newtlaws/lesson-3/double-trouble
    When you push an objects it pushes back with the same force as an attempt to move it (Newton third law). So when you treat each object individually there should be a force acting leftward which in your case you will treat it as N ( Us * N)
    So the force overall should be:
    F - n = m a

    Now after you figure things out you will end up with the answer.
  7. Feb 20, 2016 #6
    Thank you for that link. For some reason, Newton's third law did not occur to me.
    for m -> Fnet = ma = FA - FC (contact force)
    for M -> Fnet = Ma = Fc
    for m in y-direction -> Fnet = may = Fs - mg = 0
    Fs = μsFC = mg
    FC = 412.63

    Plugging the value of FC back into the net force equation for M yields a = 4.688m/s2 and using this information
    Fnet = (16)(4.688) = FA - 412.63
    FA = 487.65N

    I finally get it :)
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