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Homework Help: Kinetic energy of bow and arrow

  1. Nov 27, 2008 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    The question is if a meter stick is drawn in a bow so that it has 40 J of potential energy. When fired horizontally, the meter stick will have kinetic energy of _____.

    A) Less than 40 J B) More than 40 J C) 40 J

    I think it's C because of the conversation law. Energy can't be created or destroyed, but transformed. The energy is changing from potential energy to kinetic energy, but then I say it might be A.

    The Winterchester Potato Cannon applies a 6000N average force to a 0.5 kg potato while the potato is in the 1.5 meter barrel. Assuming no friction, what is the change in KE of the potato?

    A) 9000 J B) 6000 J C) 0J D)4500 J E) 4000J

    For this question, I'm extremely confused. I don't even know what it's asking.

    A pendulum is lifted so that it is raised 5 meter above its lowest position. When released, what is its maximum speed?

    A) 5 m/s B) 7.07 m/s C) 10 m/3 D) 100 m/s E) need to know the mass

    For this question, I think you have to know the mass because the formula is KE = 1/2mv^2. How can you figure out the maximum speed without the mass.

    Last question, which of the following are not units of Power?

    A) Nm/s B) J/s C) kgm2/s3 D) W E) Jm/s

    I think it's C and E. I just wanted to double check just to make sure that answer was correct.
  2. jcsd
  3. Nov 27, 2008 #2
    1. C seems correct if P.E is attributed to the drawing of the bow alone.

    2. Hint: Work Done = Force * Distance

    3. mgh = 1/2mv^2, do the masses matter now?

    4. Check again

    Oh and welcome to PF :)
  4. Nov 27, 2008 #3
    Thank you, that helped.

    For 1) I got C.

    2) A

    C) C

    and still working on 4. =)
  5. Nov 27, 2008 #4
    and the last one is E. =]
  6. Nov 27, 2008 #5
    Looks good to me :)
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