1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What does KE equal in this impossible physics problem?

  1. Oct 31, 2012 #1
    Help.png

    c, d, and e I don't know how to solve

    For d I tried doing

    vf2 = vi2 +2ad
    Vf = √(2*9.8*2.3) = 6.71 m/s

    Assuming the inclined plane is frictionless
    Since m2 is roped to m1, it's velocity up the ramp will be 6.71m/s too

    The total increase in KE is then 0.5[8+6]*6.71^2 = 315.16 J

    ^

    "INCORRECT"

    Right now I need a genius to tell me how to get the answer ASAP -.-

    Since (d) is wrong, so is e. -.-

    -----------------------------------

    EDIT:

    (c)

    m1 falls 2.3m and therefore has -6*9.8*2.3 = -135.24 J difference.

    m2's PE increases 8*9.8*sin 30°*2.3 = +90.16 J difference.

    The total change then is 90.16 - 135.24 = -45.08 J

    Ok so how does this relate to part d and e though? How would I solve for those two last ones?

    (d)

    KE = -PE
    KE = -mv????????

    (e)
     
  2. jcsd
  3. Oct 31, 2012 #2

    gneill

    User Avatar

    Staff: Mentor

    You pinned down the change in potential energy in your EDIT, so that's good.

    Since energy is conserved, the change in PE must be balanced by an equivalent change in KE (equal in magnitude, opposite in sign).

    What's the formula for KE of a moving mass? How much mass is moving in the problem?
     
  4. Oct 31, 2012 #3
    KE = 1/2mv2

    And m1 = 6 kg is moving..

    What next?
     
  5. Oct 31, 2012 #4

    gneill

    User Avatar

    Staff: Mentor

    So you're saying that m2 is not moving? Isn't it connected to m1 via a rope? Don't they move at the same time? Didn't the gravitational potential of both change?
     
  6. Oct 31, 2012 #5
    Oh yeah m1 and m2 are both moving.
    And yes (?) the gravitational potential changed (?)

    I need to put the conceptual stuff into a formula tho and find the total KE of the system though.... :(
     
  7. Oct 31, 2012 #6

    gneill

    User Avatar

    Staff: Mentor

    :confused: You already did that when you found the change in PE. Conservation of energy: the sum PE + KE is a constant. If PE goes down, KE goes up by the same amount.
     
  8. Oct 31, 2012 #7
    So KE = mv???
     
  9. Oct 31, 2012 #8

    gneill

    User Avatar

    Staff: Mentor

    Nooooo! mv is momentum.

    You've already found the system's change in PE. The change in KE must be equal in magnitude (but opposite in sign).
     
  10. Oct 31, 2012 #9
    So the answer is:

    ΔKE = -ΔPE
    ΔKE = -(-45.08)
    ΔKE = 45.08

    ??
     
  11. Oct 31, 2012 #10

    gneill

    User Avatar

    Staff: Mentor

    So what is the total mass that's moving?
    Right, the gravitational potential of BOTH masses changed (yielding a net change in total gravitational potential energy). But the important thing is that BOTH masses are moving.
    PE + KE = constant.

    So ΔKE = -ΔPE
     
  12. Oct 31, 2012 #11

    gneill

    User Avatar

    Staff: Mentor

    Yes. Surely the underlying theme of the chapter you're currently studying must be energy conservation for gravitational PE and KE?
     
  13. Oct 31, 2012 #12
    Ohh ok...
    And yeah it is!

    So what about the last one, part (e) for final velocity of the system?

    ΔKE = -ΔPE
    ΔKE = -(45.08)
    ΔKE = 45.08
    0.5mv2 = 45.08
    0.5(m1 + m2)v2 = 45.08
    0.5(6+8)v2 = 45.08
    7v2 = 45.08
    v2 = 6.44
    v = 2.53 m/s ??
     
  14. Oct 31, 2012 #13

    gneill

    User Avatar

    Staff: Mentor

    Yes, that looks good.
     
  15. Oct 31, 2012 #14
    Thanks!!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: What does KE equal in this impossible physics problem?
  1. Impossible Problem (Replies: 2)

Loading...