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Homework Help: Not neglecting friction force

  1. Mar 16, 2010 #1
    1. The problem statement, all variables and given/known data
    I have a homework question where it asks, "after the spring is compressed and the popgun fired, to what height does the projectile rise from point B? The question is related to an example problem in the book, where the example tells you to solve for "k", which is the spring force constant. By looking at the example i can find out what the equation for the height would be equal to, but i do not know where friction force comes into play. In the previous example it says, neglecting all resistive forces, determine the spring constant and in this example your determining height, but this time there is a resistive force (aka friction force). So, what do i do? Do i just minus the friction force.

    equation for the height is

    Yc= [MGYa+1/2KX^2]/MG where the lowercase letters are subscripts and Yc is the height that rises above point B.

    So, my question is where does friction force go into this equation so that i find the height and so that im not neglecting resistive forces(friction force)??????

    Last edited: Mar 17, 2010
  2. jcsd
  3. Mar 16, 2010 #2


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    Homework Helper

    Can you give the complete question? Where is the friction force: in the gun or in the air (as air resistance)? Is it a constant force?
  4. Mar 16, 2010 #3
    it is hard to tell you the full problem because it relates to another problem but here is the full question from the book.

    Consider the popgun in Example 8.3. Suppose the projectile mass, compression distance, and spring constant remain the same as given or calculated in the example. Suppose, however, there is a friction force of magnitude 2.00N acting on the projectile as it rubs against the interior of the barrel. The vertical length from point A to the end of teh barrel is 0.600 m (a) After the spring is compressed and the popgun fired, to what height does th projectile rise above point B? (b) draw 4 energy bar charts for this situation, analogous to those in Figures 8.6c-d

    spring constant from the example k=958N/m
  5. Mar 16, 2010 #4
    so, sorry for not answering your question but the friction force is in the barrel of the gun
  6. Mar 16, 2010 #5
    and if you're wondering where i got that height equation, the example provided in the book had k (spring force constant) equal to some letters and i just switched it around so that i got Yc= ......

    any help would be appreciated
  7. Mar 17, 2010 #6
    i need to figure this one out by tomorrow, someone help please
  8. Mar 17, 2010 #7


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    Try using the conservation of energy. The energy the popgun+bullet has at the start must equal the final energy of the popgun+bullet system, minus the energy lost by friction (which would just be W=Fd).
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