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: Spring friction physics

  1. Mar 11, 2010 #1
    1. The problem statement, all variables and given/known data
    A 50 g ice cube can slide without friction up and down a 30 degree slope. The ice cube is pressed against a spring at the bottom of the slope, compressing the spring 10 cm. The spring constant is 25 N/m. When the ice cube is released, what total distance will it travel up the slope before reversing direction?

    2. Relevant equations
    F = ma
    W = Fdx
    Ws = -1/2kx^2

    3. The attempt at a solution
    Ws = -1/2(25)(.1)^2 = 0.125J
    V1 = [2(.125/.5)]^1/2 = .7071 m/s
    F = .5(9.8)(sin30) = 2.45N
    2.45/.5 = 4.9 m/s^2

    (.7071)^2 = 2(4.9)ds
    ds = .0510m = 5.1cm

    apparently I'm incorrect in my reasoning...
    Last edited: Mar 11, 2010
  2. jcsd
  3. Mar 11, 2010 #2
    Re: springs

    When the spring decompresses there is change in K.E as well as P.E. You have considered only change in K.E. Rather look at the end positions that concern you most - the elastic P.E at maximum compression will be equal to G.P.E when block comes to rest momentarily at highest pt.
  4. Mar 11, 2010 #3
    Re: springs

    so what is the equation for P.E. then, cause I thought it was -1/2kx^2...

    So... EPIC fail... 5g = .05kg

    My answer WAS correct, the decimal place was just off by one...
    Last edited: Mar 11, 2010
  5. Mar 11, 2010 #4
    Re: springs

    I am not sure what you mean by that. But, your solution looks reasonable. There is an easier way to do it though. simply set [tex]U_{s}=U_{g}[/tex] without the kinetic energy.
  6. Mar 11, 2010 #5
    Re: springs

    By the way, 50g=0.05kg, not 0.5kg.
  7. Mar 11, 2010 #6
    Re: springs

    Dear freak_boy I wasn't referring to elastic potential energy rather it was gravitational potential energy . That is why your answer was off by a small margin - the change of height you ignored was small. Nonetheless it is important that you do not ignore it. That is why benhou's method is better.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook