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: Weight component

  1. Mar 17, 2010 #1
    1. The problem statement, all variables and given/known data
    Well I am trying to get help on weight compent. It is a box sliding on an inclined surface with angle 30 degrees.

    2. Relevant equations

    w= mg

    3. The attempt at a solution

    I am trying to find the components for weight force acting on the box. Book says w=mgcos(30) is when its perpendicular to surface, and w= mgsin(30) when its parallel to surface. Can someone explain how to do this?
  2. jcsd
  3. Mar 17, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    spdowind,welcome to PF! Are you familiar with the method for calculating vector components by drawing a right triangle? Draw a sketch. The weight force acts straight down vertically....that's the hypotenuse of the right triangle. The components are the magnitudes of the legs of that right triangle, one leg parallel to the plane surface, and the other perperpendicular to it. You'll have to do a little geometry and some basic trig to calculate these components. Please show some attempt at this for further assistance.
  4. Mar 18, 2010 #3
    See i am trying to find the horizontal forces for the block sliding down.
    I did:
    Fx= m* ax
    mg*sin(angle)- Tension force - Friction Force= mass * Acceleration.

    I just dont understand why I use mg* sin ( angle) instead of mg*cos(angle) when I do the weight x-component minus tension force and friction force.
  5. Mar 18, 2010 #4
    well from my understand again,my graph is a lil off the angle is wrong.
    What I do understand is:

    The weight is split into 2 components like my graph, but one is horizontal to the sliding surface , one is perpendicular to it, the one horizontal is the X compoent, which means parallel to surface means mgsin(angle)? Am I on the right track? Because I set up my x-y coordinates horizontal to sliding surface and perpendicular to surface.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook