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Physics problem - slingshot

  1. Jan 8, 2010 #1
    1. The problem statement, all variables and given/known data

    A slingshot consists of a light leather cup, containing a stone, that is pulled back against two parallel rubber bands. It takes a force of 15 N to stretch either one of these bands 1.0 cm.

    (a) What is the potential energy stored in the two bands together when a 50-g stone is placed in the cup and pulled back 0.20 m from the equilibrium position?

    (b) With what speed does the stone leave the slingshot?

    The answers are a) PE = 60 J b) v = 49 m/s

    2. Relevant equations

    F=kx PE=1/2kx^2

    3. The attempt at a solution

    15 = k(0.01) → k=1500 N/m

    PE = 1/2 k x^2
    = 1/2 (1500)(.20)2
    = 30 J

    PE1=KE1, KE = 1/2mv^2
    1/2mv^2 = 30
    1/2(0.050)v^2 = 30;
    v= 34.6 m/s

    Any comment or help would be appreciated about why I got the answers wrong. Thank you.
  2. jcsd
  3. Jan 8, 2010 #2
    Ok, I realized what was wrong with the first question. Since there are two straps, 30 must be doubled.
  4. Jan 8, 2010 #3
    Just remember, although you fixed your result for part A, you use it to answer part B. (In other words you have to make changes to part B as well)
  5. Jan 8, 2010 #4
    Thank you so much. :)
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