Results 1 to 4 of 4

Math Help - Energy question

  1. #1
    s3a
    s3a is offline
    Super Member
    Joined
    Nov 2008
    Posts
    597

    Energy question

    Question:
    A moving 4.5 kg block collides with a horizontal spring whose spring constant is 424 N/m (see figure). The block compresses the spring a maximum distance of 13.0 cm from its rest position. The coefficient of kinetic friction between the block and the horizontal surface is 0.36. (a) What is the work done by the spring in bringing the block to rest?


    (a) 0.564668J (THIS ANSWER IS WRONG)

    (b) How much work was done by the force of friction (non-conservative force = Wnc) while the block is being brought to rest by the spring?

    (b) -2.06388J (THIS ANSWER IS CORRECT)

    (c) What is the speed of the block when it hits the spring?

    (c) 0.50096 m/s (THIS ANSWER IS WRONG)

    What I did:
    I did f*(deltaX)*(cos180deg) = deltaE where Uf = 1/2 * k * (deltaX)^2, Ui = 0, Kf = 0, Ki = 1/2 * m * v^2 and Ef = Uf + Kf and Ei = Ui + Ki where:

    U: potential energy
    K: kinetic energy
    i: initial
    f: final

    (just in case it wasn't obvious)

    The reason for which I did not use momentum even though this seems to be an inelastic collision is because there is no mass of spring given so I am guessing that that is to be neglected. Can someone please tell me what I am doing wrong and show me how to do it?

    Any help would be greatly appreciated!
    Thanks in advance!
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    skeeter's Avatar
    Joined
    Jun 2008
    From
    North Texas
    Posts
    11,698
    Thanks
    454
    Quote Originally Posted by s3a View Post
    Question:
    A moving 4.5 kg block collides with a horizontal spring whose spring constant is 424 N/m (see figure). The block compresses the spring a maximum distance of 13.0 cm from its rest position. The coefficient of kinetic friction between the block and the horizontal surface is 0.36. (a) What is the work done by the spring in bringing the block to rest?


    (a) 0.564668J (THIS ANSWER IS WRONG)

    \textcolor{red}{|W_s| = \frac{1}{2}kx^2}

    (b) How much work was done by the force of friction (non-conservative force = Wnc) while the block is being brought to rest by the spring?

    (b) -2.06388J (THIS ANSWER IS CORRECT)

    (c) What is the speed of the block when it hits the spring?

    \textcolor{red}{KE = \frac{1}{2}mv^2 = |W_s| + |W_f|}

    (c) 0.50096 m/s (THIS ANSWER IS WRONG)

    ...
    Follow Math Help Forum on Facebook and Google+

  3. #3
    s3a
    s3a is offline
    Super Member
    Joined
    Nov 2008
    Posts
    597
    I get both wrong by using those formulae. I also tried something else and got it wrong again.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor
    skeeter's Avatar
    Joined
    Jun 2008
    From
    North Texas
    Posts
    11,698
    Thanks
    454
    Quote Originally Posted by s3a View Post
    I get both wrong by using those formulae. I also tried something else and got it wrong again.
    the formulas are correct.

    W_s = -3.58 \, J

    v = 1.58 \, m/s
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. [SOLVED] Generalised energy and energy (lagrangian mechanics)?
    Posted in the Advanced Applied Math Forum
    Replies: 2
    Last Post: January 16th 2012, 03:03 AM
  2. Energy Question
    Posted in the Math Topics Forum
    Replies: 2
    Last Post: April 22nd 2011, 09:57 AM
  3. energy question
    Posted in the Math Topics Forum
    Replies: 0
    Last Post: February 8th 2010, 05:06 PM
  4. Energy Conservation question
    Posted in the Advanced Applied Math Forum
    Replies: 1
    Last Post: March 11th 2008, 06:24 PM
  5. Replies: 7
    Last Post: April 17th 2006, 01:04 PM

Search Tags


/mathhelpforum @mathhelpforum