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!

Homework Help: Pinball Machine

  1. Dec 10, 2006 #1
    1. The problem statement, all variables and given/known data
    The ball launcher in a pinball machine has a spring that has a force constant of 1.2 N/cm. The surface on which the ball moves is inclined at 10° with respect to the horizontal. If the spring is initially compressed 5 cm, find the launching speed of a .1 kg ball when the plunger is released. Friction and the mass of the plunger are negligible.

    2. Relevant equations
    Ue (Elastic potential) = (1/2)kx^2
    K (Kinetic) = (1/2)mv^2
    Ug (Gravitational potential) = mgh

    3. The attempt at a solution

    k = 1.2 N/ cm = .012 N / m
    x = 5 cm = .05 m
    m = .1 kg
    g = 9.8 m/s

    Ue = K + Ug

    .5kx^2 = .5mv^2 + mgh

    .5(.012)(.05^2) = .5(.1)v^2 + .1(9.8)(.05sin(10°))

    0.000015 = .05v^2 + .00851

    At this point, my v^2 will equal a negative number, which makes no sense at all. I'm stuck >.<

    The problem is that the left side of the equation gets exponentially smaller while the right hand side... doesn't.

    Edit: Foolish calculator mistake. I hate how I find this out right after I posted it (I checked my work for 1/2 hr before posting).

    1.2 N / cm = 120 N / m, not .012. I hit * instead of / on my calc.
    Last edited: Dec 10, 2006
  2. jcsd
  3. Dec 10, 2006 #2
    check your conversion from n/cm to n/m
  4. Dec 10, 2006 #3
    I think it's just your conversion for the k value; you have to multiply
    1.2N/cm*100cm/1m = 120N/m

    my bad... didnt see that someone already replied.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook