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Homework Help: I really need help, final tomorrow lol

  1. Mar 19, 2009 #1
    1. The problem statement, all variables and given/known data
    1. A woman weighing 730N stands in the middle of a frozen pond of radius 5m. She is unable
    to get to the other side because of a lack of friction between her shoes and the ice. To propel
    herself, she throws a 1.2kg backpack horizontally towards the south shore at a speed of 5m/s.
    How long does it take her to reach the north shore?

    2. Two blocks are in contact on a frictionless surface., sitting horizontally next to each other. You apply a force of 3.2N to block 1 such that it pushes against block two. Mass of block 1 = 3.2kg. Mass of block 2 = 1.2 kg. What is the magnitude of the force between the two blocks?

    3.The density of bubble gum is about 1g per cubic cm. You blow an 8g wad of gum into a
    bubble with a 10cm diameter. What is the thickness of the bubble's skin?

    2. Relevant equations
    1. conservation of momentum (explosion?) (Mass A + Mass B)V initial = Mass A*V A + Mass B * V B?
    2. F=ma F=0
    3. no idea? 4/3pi r^3?

    3. The attempt at a solution
    I did number 1, my answer came out to be something like 62.07 seconds but i am not sure if its right. Need someone to check for me. (I set the initial velocity to zero since the backpack on the woman AND the woman were in the middle of the pond not moving right?)

    number 2 and 3 i have no idea, i drew the diagrams out but does the box being pushed on have a normal force pushing back or no?

    number 3 i just have absolutely NO idea lol
  2. jcsd
  3. Mar 19, 2009 #2


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    Homework Helper

    Problem 1 is correct.
    For problem 2, draw the free body diagram. Normal forces on the blocks are balanced by the weights of the blocks.
    Consider action and reaction forces acting on block 2 and block one.
    Both the blocks must move with the same acceleration.
    In problem 3, Surface area of the sphere is given by A = 4*pi*r^2. If t is the thickness of the skin, find the volume. Volume X density = mass of the bubble gum.
  4. Mar 20, 2009 #3
    I got everything now cept for number 3. Ok so i find the surface area and i have the volume, so we find the thickness of what? the bubble or the bubble gum? >.<
  5. Mar 20, 2009 #4


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    Homework Helper

    Mass of the bubble gum = Area of the bubble*thickness of the bubble*density of the bubble gum
  6. Mar 20, 2009 #5
    i did that but the answer came out to be .025 CM but the answer is .02 mm on the practice so i am confused
  7. Mar 20, 2009 #6
    I think the teacher said something about subtracting the outer layer with inner layer or something like that
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