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: Going down a slide CONSERVATION OF ENERGY

  1. Aug 26, 2010 #1
    1. The problem statement, all variables and given/known data
    Suppose a slide is 35.0 meters high, but is a straight slope, inclined at 45 degrees with respect to the horizontal.

    a) find the speed of a 60.0kg thrill seeker at the bottom of the slide, assuming no friction.
    b) if the thrill seeker has a speed of 20.0m/s at the bottom, find the change in mechanical energy due to friction
    c) find the magnitude of the force of friction

    2. Relevant equations
    KEi + PEi = KEf + PEf

    3. The attempt at a solution

    a) since there is no friction, mechanical energy is conserved. So therefore using the above equation: KEi + PEi = KEf + PEf. However, i have difficulty when the question adds an angle. When do I know when to break into x and y components??

    In this case, KEi = 0, PEi = mgy1, KEf = 1/2mv^2, PEf = 0

    Therefore, PEi = KEf
    mgy1 = 1/2mv2^2
    square root of 2gy1 = v2^2 (where y1 = 35m)

    is that right?
  2. jcsd
  3. Aug 26, 2010 #2
    Your notation is a little confusing, but from your set up you want to solve for v in the following equation

  4. Aug 26, 2010 #3


    User Avatar
    Science Advisor

    That's a great thing about the energy method of solving problems like this. Energy is energy. It doesn't matter how you get from State 1 to State 2. You can largely ignore what happens in between.

    Now, in "real-life", there will always be friction. The friction is defined as the normal component of the weight, multiplied by the friction coefficient. That means that the shallower the angle, the greater the friction force, and the slower resulting speed. However, neglecting friction, you can ignore that fact.
  5. Aug 26, 2010 #4
    So when do you consider the angles? Only when there is friction?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook