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

Negative work and energy

  1. Oct 27, 2013 #1
    How can we define negative work to be? I can't get my head around negative work or energy. However, I'm thinking that it must be work that is done opposite in the direction of motion of a particle or body (say, friction). Could someone please help me understand this concept?
     
  2. jcsd
  3. Oct 27, 2013 #2
    Yeah. I'll give you a simple example hopefully you understand. If you pick up and object with weight you are doing negative work against gravity. If you let go of the object gravity is doing positive work.
     
  4. Oct 28, 2013 #3

    jtbell

    User Avatar

    Staff: Mentor

    No, in this case the work you do is positive. The force you exert and the direction of motion are in the same direction: both upwards.

    If you lower an object while holding it in your hand, then you do negative work on the object. In this case the force is still upwards, but the direction of motion is downwards, i.e. in opposite directions.

    Another way to look at it is that "you do negative work on an object" is equivalent to "the object does positive work on you." Practical application: the weight that drives a grandfather clock. The clock does negative work on the weight, and the weight does positive work on the clock, as the weight is slowly lowered.
     
  5. Oct 28, 2013 #4
    Have a look..
     

    Attached Files:

  6. Oct 28, 2013 #5
    That's not right. The weight does no work on the clock because the clock is not moving. The law of action and reaction doesn't transfer from forces to work because the work also depend on whether or not the object on which the force acts is moving (displacement).
     
  7. Oct 28, 2013 #6

    Nugatory

    User Avatar

    Staff: Mentor

    Suppose I changed jtbell's statement to add the text in boldface above... then it would be right (and I expect that's what he intended anyways).
     
  8. Oct 28, 2013 #7

    jtbell

    User Avatar

    Staff: Mentor

    Yes, the clock as a whole does not move, so I made a poor choice of words.
     
  9. Oct 28, 2013 #8
    So what was wrong with my statement again?
     
  10. Oct 28, 2013 #9

    Doc Al

    User Avatar

    Staff: Mentor

    Since the force you exert and the displacement are both upward, you are doing positive work.

    When the object falls, gravity does positive work.
     
  11. Oct 28, 2013 #10
    But doesn't it depend what context you say you're doing work? The example I gave was relative to gravity. You are saying exactly what I'm saying. I said and you said gravity does positive work when letting go. Okay that is settled. Now if you pick up an object you are doing positive work. Another way to say that which has the same physical meaning is that picking up an object is equivalent to gravity doing negative work on the object since gravity is a conservative force. I do positive work, simultaneously gravity does negative work. Gravity does positive work, simultaneously I do negative work when lowering down the object. I know this because I read three different texts and saw Walter Lewin explain it in his classical mechanics lecture. Do you see what I am saying?
     
  12. Oct 28, 2013 #11

    Doc Al

    User Avatar

    Staff: Mentor

    All good.

    So why didn't you say that the first time? :wink: Read what you actually wrote earlier.

    You said that when you pick up an object you do negative work against gravity, which was incorrect. As you said correctly in your last post, you do positive work against gravity.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Negative work and energy
  1. Negative energy (Replies: 3)

  2. Negative Energy (Replies: 3)

  3. Negative Energy (Replies: 1)

Loading...