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Robert100
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No "work" done (apparent paradox) What am I missing?
I have a simple question about work. Work is force times distance, in the direction that the force is applied. So when I carry a box, at constant velocity, at constant height, we draw a free-body diagram like that shown below. Since the force applied is perpendicular to the motion, we say that the total work done on the box is zero. (See figure.)
http://www.nu.ac.za/physics/1M2002/Energy%20work%20and%20power_files/image004.jpg"
[link fixed]
Here is my problem: Why are we only looking at one force (vertical)? Isn't there a horizontal friction force? Why is this force not considered?
I know that we don't change the box's velocity; we don't change its PE or KE. So if no change in energy occurs, no work is done. Fine. But why don't any textbooks mention the existence of a sideways friction force (without which the box would not move?) I get the feeling that the textbooks are making an over-simplification, and leaving out essential need to know information.
I have noticed some very bad problems shown in otherwise good books. Books say that in a given problem, a body in motion across a tabletop requires zero force to stay in motion. In book after book, test after test, I see this, without the provisio being given that this is only true when no friction exists.
When friction exists a force must be applied continuously (to counter the friction force, so the net force is zero.) Usually the textbooks get it correct in one paragraph, but then explain it wrong everywhere else (and also in most test questions.)
Is it no wonder that so many of our students get some answers right, but revert to Aristotelian physics when they try to understand a situation?
What am I missing in the original question about work? How is it that we do zero work, even though we are applying a horizontal force (which is in the direction of motion)
Robert
I have a simple question about work. Work is force times distance, in the direction that the force is applied. So when I carry a box, at constant velocity, at constant height, we draw a free-body diagram like that shown below. Since the force applied is perpendicular to the motion, we say that the total work done on the box is zero. (See figure.)
http://www.nu.ac.za/physics/1M2002/Energy%20work%20and%20power_files/image004.jpg"
[link fixed]
Here is my problem: Why are we only looking at one force (vertical)? Isn't there a horizontal friction force? Why is this force not considered?
I know that we don't change the box's velocity; we don't change its PE or KE. So if no change in energy occurs, no work is done. Fine. But why don't any textbooks mention the existence of a sideways friction force (without which the box would not move?) I get the feeling that the textbooks are making an over-simplification, and leaving out essential need to know information.
I have noticed some very bad problems shown in otherwise good books. Books say that in a given problem, a body in motion across a tabletop requires zero force to stay in motion. In book after book, test after test, I see this, without the provisio being given that this is only true when no friction exists.
When friction exists a force must be applied continuously (to counter the friction force, so the net force is zero.) Usually the textbooks get it correct in one paragraph, but then explain it wrong everywhere else (and also in most test questions.)
Is it no wonder that so many of our students get some answers right, but revert to Aristotelian physics when they try to understand a situation?
What am I missing in the original question about work? How is it that we do zero work, even though we are applying a horizontal force (which is in the direction of motion)
Robert
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