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Homework Help: Spring Work and Energy

  1. Dec 20, 2013 #1
    ImageUploadedByPhysics Forums1387563483.653939.jpg

    The rest if the question ask me to find the the point at which the ball will have maximum speed ?

    I don't know how to approach it exactly?
    From my conceptualization I can see that the maximum will be when the spring returns to the uncompressed position at 5.0cm. However the answers disagree and they claim its 4.6 cm ?

    Please help me how did he reach this conclusion ?
  2. jcsd
  3. Dec 20, 2013 #2
    Think about the work energy theorem.

    Does that help?
  4. Dec 20, 2013 #3
    Not really 2 unknowns !
    No other equations !
    ImageUploadedByPhysics Forums1387566332.576675.jpg
  5. Dec 20, 2013 #4
    You actually know x here. as it says in the problem: "the ball moves 15 cm through the barrel of the canon and the barrel exerts a constant friction force of 0.032N on the ball". This 15 cm is where the friction is doing work.
  6. Dec 20, 2013 #5
    Man... My picture did not upload. Anyone else has this problem?

    Yes! Now you would get... -0.320x0.15=1/2x5.30x10^-3kgV^2-1/2x8x0.05^2

    Now... V=...
  7. Dec 20, 2013 #6
    Guys I already solved a) my problem is in b and c where he asks for the point at which velocity is maximum!
  8. Dec 20, 2013 #7
    Can you please be more specific about what b & c are?

    Is it when compression m(-x+500) is at a distance x?
  9. Dec 20, 2013 #8


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    It would help if you had actually posted parts b and c of the problem, then :rolleyes:
  10. Dec 20, 2013 #9
    I wrote the complete question in my original post !
  11. Dec 20, 2013 #10
    B) at what point will the ball have a maximum speed ?

    C) what is the maximum speed?
  12. Dec 20, 2013 #11


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    The ball would have the maximum speed when it just leaves from the toy,without moving any of the 15cm of the cannon.

    Using F=ma and F=ke --> k=Force constant e =extension
    Find force.
    Using F=ma,Find acceleration
    Then you can know the speed
  13. Dec 20, 2013 #12


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    Don't give me your indignation, man. You were supposed to use the template for posting homework help threads, the one that requires an *exact* problem statement and an attempt at the solution. You know, the template that you deleted when you made your original post, in spite of being asked in boldface red text to use it? Didn't you see this above the compose window?


    So your post is in violation of PF rules, and by all rights I should delete it. But I'll let you off this time.

    The above would be true if there were no friction. But there is friction, which does work, removing kinetic energy from the system. So your expression for the conservation of energy must include the work done by friction. If you write down the conservation of energy in this way, you will end up for an expression for the kinetic energy vs. position. From that, it shouldn't be too hard to solve for the position that maximizes the kinetic energy.

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