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Work and energy probelms

  1. Oct 19, 2005 #1
    1.A 50.3-g golf ball is driven from the tee with an initial speed of 40.6 m/s and rises to a height of 29.9 m. (a) Neglect air resistance and determine the kinetic energy of the ball at its highest point. (b) What is its speed when it is 6.17 m below its highest point?

    a. KE=1/2mv^2
    i used this formula to find the KE but i keep getting the wrong anwer
    KE=1/2(0.0503 kg)(40.6)^2=41.5 but this is the wrong answer. i;m not sure why it's wrong...
    b. i used the formula:
    KEinitial+PEinitial=KEfinal+PEfinal
    PE initial=0 cause the initial height is 0 and KE final is 0 because the final velocity is 0. so the i get: KEinitial=PEfinal and for final height i get 11. but i don't think that i did this correctly though

    2. A cyclist approaches the bottom of a gradual hill at a speed of 27.0 m/s. The hill is 7.29 m high, and the cyclist estimates that she is going fast enough to coast up and over it without pedaling. Ignoring air resistance and friction, find the speed at which the cyclist crests the hill.

    i used KEinitial+PEinitial=KEfinal+PEfinal and solved for the final velocity and i got 14.8 but that is wrong

    3. A person is making homemade ice cream. She exerts a force of magnitude 26.4 N on the free end of the crank handle, and this end moves in a circular path of radius 0.271 m. The force is always applied parallel to the motion of the handle. If the handle is turned once every 1.16 s, what is the average power being expended?

    i think you would use the equation P=Work/Time in this problem but other than that, i'm not sure where to start.

    any help on any of the problems would be great. thanks so much
     
  2. jcsd
  3. Oct 19, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What you've calculated is just the initial KE of the ball. Use conservation of energy to find the KE at the highest point.
    Careful! The KE would be zero at the highest point only if the ball were hit straight up. (That's why they gave you its maximum height in order to solve part a. ) But you can use conservation of energy.
    The idea is correct. Show the details of your work and we can find the problem.
    Hint: The work is force X distance. What's the work done to turn the handle once?
     
  4. Oct 19, 2005 #3
    1.A 50.3-g golf ball is driven from the tee with an initial speed of 40.6 m/s and rises to a height of 29.9 m. (a) Neglect air resistance and determine the kinetic energy of the ball at its highest point. (b) What is its speed when it is 6.17 m below its highest point?

    the KE at the highest point is...

    KEi + PEi = KEf + PEf

    PEf = mgh
    PEi = 0
    KEi= 1/2mv^2
    KEf= 1/2mvf^2

    replace thse formulas and solve for KEf....

    and b is the same thing differnet heights... good luck
     
  5. Oct 19, 2005 #4
    thanks for the help guys but i'm still having a little troble
    1. A 50.3-g golf ball is driven from the tee with an initial speed of 40.6 m/s and rises to a height of 29.9 m. (a) Neglect air resistance and determine the kinetic energy of the ball at its highest point. (b) What is its speed when it is 6.17 m below its highest point?

    i got the correct answer for part b but i'm having troble with part a
    i used:
    KEi+PEi=KEf+PEf
    PEi=0 so it's KEi=KEf+PEf
    1/2(40.6)^2=KEf+(9.8)(29.9)
    KE=531.16
    what did i do wrong?

    2. A cyclist approaches the bottom of a gradual hill at a speed of 27.0 m/s. The hill is 7.29 m high, and the cyclist estimates that she is going fast enough to coast up and over it without pedaling. Ignoring air resistance and friction, find the speed at which the cyclist crests the hill.

    for this problem i did:
    vf=square root(Vo^2+2g(Hinitial-Hfinal)
    =square root(27^2+2(9.8)(7.29)) and i get 29.5 which is the wrong answer
     
  6. Oct 19, 2005 #5
    1/2(40.6)^2

    ? where is your mass?

    1/2 m v^2


    +(9.8)(29.9)

    again , where is ur mass, u cant just cancel them out because you dont have a mass term on your KEf ... well you would if you break it down but you should not break it down.... calculate it with the masses
     
  7. Oct 19, 2005 #6
    oh,i forgot the mass. i usually don't include the mass because they have have it and i just cancel it out. thanks for the help
     
  8. Oct 20, 2005 #7

    Doc Al

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    Staff: Mentor

    Hinitial-Hfinal is negative.
     
  9. Oct 20, 2005 #8
    thanks again for the help
     
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