Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Positive/negative work

  1. Aug 31, 2005 #1
    lets say you have a handsaw and you a sawing a tree by pushing the saw away from you and pulling it toward you. you are doing work when you push and when you pull the saw but when is the work positive or negative. or is the work positive during both the pushing and pulling
  2. jcsd
  3. Aug 31, 2005 #2
    Its positive both ways, since you are losing energy both times.
  4. Sep 1, 2005 #3
    The easy way to know if you ae doing poitive or negative work is to look for solving the expression W=F.s , in the above case , since you apply force forwards and the saw displaces forward , so the dot product will be poitive sice cos0=1 , and same is the case when you pull the saw towards yourself.

  5. Sep 1, 2005 #4


    User Avatar
    Homework Helper

    Work is a scalar quantity. Here is + and - sign gives whether the work is done by that force or done against the force i.e. whether the energy is supplied by the system applying force of the energy is supplied to the system

    In your example the work done by you is positive in both cases but work done by the force applied by the tree on the saw is negative.
  6. Sep 2, 2005 #5


    User Avatar
    Homework Helper

    Work done by a force

    Raikou Tatsu, The force in we are talking about is the one that you are exerting on the saw when you are pushing it away from you , Ff, and pulling it towards you, Fb. In both cases the displacement of the saw is in the same direction as the applied force. Therefore the work done is positive in both case - see the attached drawing.

    Attached Files:

    • work.bmp
      File size:
      13.7 KB
  7. Sep 2, 2005 #6
    To remember if it's + or -,
    + when the force and the displacement are in the same direction.
    - when the force and the displace are in opposite direction.
    Sor your answer must +
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook