Is Work on a Static Object Possible?

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Discussion Overview

The discussion revolves around the concept of work in physics, particularly in scenarios involving static and dynamic objects. Participants explore definitions of work, its calculation in different contexts, and the implications of forces like gravity and friction.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that if an object does not move, no mechanical work is done on it, leading to a conclusion of zero work.
  • Others propose that work can be calculated using the equation W=w*d*cos(theta) or W=w*d*cos(90-theta) when considering a box sliding down a frictionless slope, with ongoing debate about which equation is appropriate.
  • A participant suggests that gravity is the active force causing the box to move down the slope, while questioning the implications of removing gravity and friction.
  • There is a discussion about the nature of weight and gravity, with some participants noting that gravity is approximately constant on Earth but can vary slightly at higher altitudes.
  • One participant mentions that if an object does not move, mechanical work is zero, but other forms of work (elastic, thermal, biological) may still be occurring, although they are typically not counted in standard problems.
  • Another participant questions the use of sine versus cosine in the equations for work, suggesting a potential simplification.

Areas of Agreement / Disagreement

Participants express differing views on the definition and calculation of work, particularly in static versus dynamic scenarios. There is no consensus on the appropriate equations for calculating work in the context of a sliding box, and the discussion remains unresolved regarding the implications of gravity and friction.

Contextual Notes

Participants reference various assumptions about forces acting on objects, the definitions of work, and the conditions under which work is calculated. There are unresolved mathematical steps and dependencies on definitions that affect the discussion.

Vivee=)
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So I'm trying to push an big object and it doesn't move, am I doing work on it? Or is work 0 joules?
 
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What is the definition of work?
 
Work is the force I would apply times the distance the object moves... OHHH, it's that simple. It doesn't move so work would be zero. Thanks cristo.
 
Ok, I have another work question. What if the box is sliding down a frictionless slope. Would the equation for work be W=w*d*cos(theta), with w=weight and d=distance? Or would it be W=w*d*cos(90-theta)? I'm guessing it's one of these because since the box is moving, work cannot be zero.
 
Vivee=) said:
Ok, I have another work question. What if the box is sliding down a frictionless slope. Would the equation for work be W=w*d*cos(theta), with w=weight and d=distance? Or would it be W=w*d*cos(90-theta)? I'm guessing it's one of these because since the box is moving, work cannot be zero.

In that case the work would be done by gravity. Calculate the force that gravity is exerting on the box at that angle and relate it to the distance.

You can think of it this way. If there was no gravity, the box would just sit at the top of the slope because nothing is pushing it down.
 
Ohh yes, I got it. It must be W=w*d*cos(90-theta) because W=w*d*cos(theta) would find something else.
 
Math Jeans said:
If there was no gravity, the box would just sit at the top of the slope because nothing is pushing it down.

What if there was no friction?
 
Colin1 said:
What if there was no friction?

Even if there is no friction, nothing is pushing the box down. Space is a vacuum, there is no air resistance. If you are in space, there is no gravity, and no friction, and you still float. So does the box.
 
Interesting
so, if

weight = mass x gravity

is gravity a constant here? If so, what varies as what now that it's been removed?
 
  • #10
Colin1 said:
Interesting
so, if

weight = mass x gravity

is gravity a constant here? If so, what varies as what now that it's been removed?

Gravity on Earth is approximately equal to 9.81 m/s^2. This changes very slightly at higher altitudes, but the change is so small that it can be left out.

The removal of gravity was just an example in order to show that it was the active force pushing the box down.
 
  • #11
Math Jeans said:
The removal of gravity was just an example in order to show that it was the active force pushing the box down.

Sure
I understood that, I was just wondering where the equation went from there
 
  • #12
Oh, I figured it out myself. Thanks for the reply.
 
  • #13
Yeah
I think I did too
you end up with mass varying as mass or something equally useful :smile:
 
  • #14
Consider that the object would essentially have to be of infinite mass and perfectly rigid not to perform work on it.
 
  • #15
You should specify that, if the object does not move, you are doing no MECHANICAL work on it. You are, however, doing some combination of elastic work, thermal work, and biological work; it's just that we don't usually count those in this kind of problem.
 
  • #16
Wouldnt it be easier to use Sin(Theta) rather then Cos(90-Theta)?

Edit: Since Cos is just a derivative function of Sin, and is 90 out of phase with Sin anyway, so adding 90 is effectively just superimposing a Cos wave onto a Sin?
 
Last edited:
  • #17
In fact scratch that, that would be +90+Theta rather then minus i guess...
 

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