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: Kinematics and also an energy

  1. Dec 17, 2011 #1
    1. The problem statement, all variables and given/known data
    1.A block is sent up a frictionless ramp along which an x axis extends upward. The figure below gives the kinetic energy of the block as a function of position x; the scale of the figure's vertical axis is set by Ks = 38.0 J. If the block's initial speed is 4.70 m/s, what is the normal force on the block?

    2. A startled armadillo leaps upward rising 0.553 m in the first 0.197 s.
    (a) What is its initial speed as it leaves the ground?
    Correct: Your answer is correct. m/s

    (b) What is its speed at the height of 0.553 m?
    Correct: Your answer is correct. m/s

    (c) How much higher does it go?



    2. Relevant equations
    F=ma
    v^2=v0^2+2ad



    3. The attempt at a solution

    For the first question

    KE=.5mv^2
    152 J = .5m(4.70 m/s)^2
    m=13.8 kg

    Height of the ramp
    .5mv^2=mgh
    152 J=13.8kg*9.8m/s^2*h
    h=1.13 m
    Solve for the angle of the ramp
    sin-1(1.13m/2m) = 34 deg
    N-mgcos34=0
    N=13.8*9.8cos34=111 N. But this is wrong.

    2nd question...
    solved pt a and b, got 3.77 m/s for v0, 1.84 m/s vf

    v0 is 0 m/s at the top of the height, so
    0=3.77m/s ^2 +2*9.8m/s^2d
    solve for d, get .725
    difference is .17 m. But this is wrong.

    Thank you so much! Final monday and I have a C-:/
     
  2. jcsd
  3. Dec 17, 2011 #2

    PeterO

    User Avatar
    Homework Helper

    Where did you get 152 Joules from? You said the energy was 38J [which only one quarter of 152]
     
  4. Dec 17, 2011 #3
    Oops! Sorry, forgot to include this image
    http://www.webassign.net/hrw/7-33.gif

    It's a scale of 38 J so, at the top (0m) E=38J(4)=152 J
     
  5. Dec 18, 2011 #4

    PeterO

    User Avatar
    Homework Helper

    Where did you get the idea it is a scale 38J?

    The problem states that Ks = 38J. That is not some sort of scale. They have not said 38J per division.

    Also this problem was the subject of another post earlier?????
     
  6. Dec 18, 2011 #5
    Wow, apparently I can't read.

    And yes, I found that post but I think my setup is different then theirs but still correct.

    I know get a value of 27.9523 N for the normal force.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook