Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: KE And PE and Work?

  1. Oct 10, 2011 #1
    1. The problem statement, all variables and given/known data
    How do you solve this??

    A 2kg block starts from rest slides down a friction-less incline plane that is at angle of 20degree with the horizontal. At a certain point on the incline, the speed of the block is 9m/s.

    1.What is the vertical distance the block has traveled??
    2.What is the work done by gravity as the block moves the vertical distance you found??





    2. Relevant equations

    Work= Force x Displacement
    KE= .5mv^2
    PE= mgh

    3. The attempt at a solution

    for number 1, I know that my PEf nd KEi =0.
    So i set PEi and KEf equal to each other.
    PEi=KEf.
    So what i did was mgh=.5*m*v^2.
    m's cancel out.

    so 9.8h=.5v^2.
    .. This is where i am stuck.. I am not sure if i am doing right.. But how do i find v which is velocity?
     
  2. jcsd
  3. Oct 10, 2011 #2

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    I think it gives the velocity in the problem statement. (actually the speed)
     
  4. Oct 10, 2011 #3
    Then how do i solve it?
     
  5. Oct 10, 2011 #4

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Solve your last equation for h.
     
  6. Oct 10, 2011 #5
    I cant..
     
  7. Oct 10, 2011 #6
    O~ do i just substitute 9??
     
  8. Oct 10, 2011 #7

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    You should be able to solve it. What is v ?
     
  9. Oct 10, 2011 #8
    the speed is 9.. Okay i got #1 which is 4.14 m..

    But cannot find the work done by gravity??
     
  10. Oct 10, 2011 #9

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    What's the force of gravity, on the block?
     
  11. Oct 10, 2011 #10
    39.24
     
  12. Oct 10, 2011 #11
    Is this right??

    W= 39.24 * 4.13cos(30).

    = 140??
     
  13. Oct 10, 2011 #12
    in joules??
     
  14. Oct 10, 2011 #13
    Or is it Pe=mgh??

    4kg*9.8*4.13= 161J???
     
  15. Oct 10, 2011 #14

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Why 4 kg ?

    (2kg )(9.8 N/kg) = ?

    The weight!
     
  16. Oct 10, 2011 #15
    Oh sorry. I thought the mass was 4kg.
    so its actually.

    2kg * 9.8 n/kg = 19.6 N.

    So what now??
     
  17. Oct 10, 2011 #16

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Work= Force x Displacement (sort of)

    Actually: Work= Force x (the component of Displacement that's in the direction of the force)

    the component of Displacement that's in the direction of the force = h
     
  18. Oct 10, 2011 #17
    so its.

    W=19.6 N * 4.13cos(20)?
     
  19. Oct 10, 2011 #18
    or is it sin(20)??
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook