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Easy conceptual energy problem?

  1. Oct 26, 2013 #1

    mot

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    "Easy" conceptual energy problem?

    1. The problem statement, all variables and given/known data
    Here's the diagram

    http://nkkkjkjjjjjjjjjjj.tumblr.com/image/65137695911
    A bead slides on a wire, which is in a vertical plane, as shown in the diagram. Gravity acts in the -y direction. The bead starts at A, moving to the right with an initial velocity v. The wire is frictionless between A and D and between F and G, but there is friction between D and F. (For each statement select True, False, Greater than, Less than, Equal to, or Not enough information to tell.
    1. The bead's kinetic energy at B is ... its kinetic energy at F
    2. Between A and C, there is positive work done on the bead
    3. Between D and F, the speed decreases, then increases
    4. The speed at C is ... the speed at E


    2. Relevant equations
    E_g=mgy
    E_k=1/2mv^2

    3. The attempt at a solution
    The bead's kinetic energy at B is ... its kinetic energy at F: For this is said greater than because, although they are at the same height, all the energy is not conserved because it experiences friction between D and F.
    Between A and C, there is positive work done on the bead
    I said True because the dispacement is in the same direction as the force of gravity
    Between D and F, the speed decreases, then increases
    I said true for this one. At D it experiences friction and slows down while moving horizontally, then it starts to go down, so a component of gravity acts to accelerate it. Even if it is still slowing down, it's still speeding up relative to its speed at E.
    The speed at C is ... the speed at E
    I put not enough info...We don't know how much friction slowed the bead down between D& E compared to how much it "sped up" on that decline between C & D.

    But these answers are wrong...can someone help? Thanks so much!
     
  2. jcsd
  3. Oct 26, 2013 #2
    Your solutions look correct to me.
    Welcome to PF
     
    Last edited: Oct 26, 2013
  4. Oct 26, 2013 #3
    Your answer to 3 looks wrong. You don't know whether gravity will win out over friction. So, it could still be slowing from E to F. Not enough info.
     
  5. Oct 26, 2013 #4

    mot

    User Avatar

    That was it! It makes sense too. The "deceleration" from E to F should be less than D to E, but it could still be slowing relative to the speed before D, or it could be speeding up depending on the angle.
    Thanks! :)
     
  6. Oct 26, 2013 #5
    agree
     
  7. Oct 26, 2013 #6
    Also note that the slope between E and F never becomes vertical, so there is always going to be a normal force. This allows a frictional force to always be present over the entire path between E and F.

    Chet
     
  8. Oct 23, 2014 #7
    the same diagram but different question, mine is
    True False Greater than Less than Equal to Not enough information to tell The bead's potential energy at B is ... its potential energy at F
    True False Greater than Less than Equal to Not enough information to tell Between A and B, the speed increases
    True False Greater than Less than Equal to Not enough information to tell The speed at C is ... the speed at E
    True False Greater than Less than Equal to Not enough information to tell Between D and E, there is positive work done on the bead,
    i choose these answers but its incorrect i don't know which one do i get wrong
     
  9. Oct 23, 2014 #8
    the same diagram but different question, mine is
    The bead's potential energy at B is ... its potential energy at F
    Between A and B, the speed increases
    The speed at C is ... the speed at E
    Between D and E, there is positive work done on the bead,
    the first one i choose greater than , second i choose true,third i choose not enough inform, the last one i choose false
    i choose these answers but its incorrect i don't know which one do i get wrong
     
  10. Oct 24, 2014 #9
    Please tell us your rationale for each of your choices. That's more important than getting the actual right answer.

    Chet
     
  11. Oct 25, 2014 #10
    i think EP B is GREATER than F is because when its the same height F experience the friction which means that the Ep is not all transferred at position F
    and i think the speed is increasing from A to B due to the conservation of energy mgh(i) + 1/2mv(i)^2= mgh(f)+1/2mv(f)^2,so as the height decrease the speed will increase.speed between C and E is not enough inform to tell since E undergoes friction but c has the larger height, and the last one i think it should be negative,because they have the same height and when undergoing the friction its a process of losing energy which will cause negative work , this is my understanding i am not sure whether it's correct or not
     
  12. Oct 25, 2014 #11

    This is the one that's wrong. Now that you know it, can you figure out why?

    Chet
     
  13. Oct 25, 2014 #12
    sorry i don't get why, wait is that because potential energy is no relation to friction since kinetic energy will be the one matters?
     
  14. Oct 25, 2014 #13
    i get it , it should be equal thank you so much
     
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