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Homework Help: Forces and Newton's Laws

  1. Jul 30, 2009 #1
    1. The problem statement, all variables and given/known data
    A cup of coffee is sitting on a table in an airplane that is flying at a constant altitude and a constant velocity. The coefficient of static friction between the cup and the table is 0.30. Suddenly, the plane accelerates, its altitude remaining constant. What is the maximum acceleration that the plane can have without the cup sliding backward on the table?

    2. Relevant equations
    fs(max) = coefficient of friction * normal force
    Normal force = Work = mass * acceleration

    3. The attempt at a solution
    It doesn't give a force, work, or mass. All I can do is look at equations with 2 or 3 unknown variables. In addition to help with this problem, does anyone have any suggestions on solving problems that seem to be missing information? I'm coming across a lot of them and there's nothing worse than being stumped before even attempting to solve.
  2. jcsd
  3. Jul 30, 2009 #2
    You can't solve this problem because you don't know the whole story. The acceleration the cup of coffee could withstand without budging is different if it weighs 10 pounds or 1,000,000 pounds.
  4. Jul 30, 2009 #3
    I think you can solve the problem. Usually in these types of problems with apparent missing values, you just go ahead and start writing out equations anyway and use a variable to represent the missing information. In this case, the mass is missing. However, if you do the force diagram and set the sum of horizontal forces = m*a, you will find that the mass cancels on each side. Usually when mass is not given, the problem is designed so that mass will cancel out. Try again and see if you get it.
  5. Jul 31, 2009 #4
    I'm confused as to why the net horizontal force would be = mass * acceleration.
    Mass * acceleration = Weight, which acts in the vertical direction.
  6. Jul 31, 2009 #5
    I just wanted to mention that the normal force does not equal work. Work is Force * Distance. FYI only.
  7. Jul 31, 2009 #6
    "Suddenly, the plane accelerates, its altitude remaining constant"

    There is a normal force on the cup from the table that balances the weight (mass * g) of the cup. However, the plane is remaining at constant height and the cup is not flying through the air, but remaining on the table, so the net acceleration of the cup in the vertical direction is zero.

    However, the plane is accelerating in the horizontal direction, so....
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