Misconceptions about Forces

[Cap'n Refsmmat]

So yes - by holding onto the string and supporting the outward force and causing the ball to go curvilinear like all good orbits you are performing work -

 

That's where you're wrong.

 

http://hyperphysics....BASE/work2.html (Georgia State University's HyperPhysics site)

 

For circular motion, the centripetal force always acts at right angles to the motion. It changes the direction of the motion, but it does no work on the object. This can be applied to any circular orbit.

 

My textbook, Physics for Engineers and Scientists, lists the definition of work as [imath]W=\mathbf{F}\cdot\mathbf{s}[/imath], where [imath]\mathbf{F}[/imath] and [imath]\mathbf{s}[/imath] are vectors representing force and displacement (distance), respectively. Work is the dot product of those two vectors. The dot product is 0 when the force is applied perpendicular to the motion of the object, as it is in circular motion (the ball is going around the circle, while the string pulls it directly towards the center.)

[gentleman-farmer]

Your analysis is incorrect when a force changes straight line motion (to curvilinear motion) the movement of the object (while it traverses) is toward the center in line with the force being applied.

 

Work is being performed to cause it to go curvilinear

 

If we were to believe your sources we'd have to give up on Newton's Universal Law of Gravitation - and I don't think anyone is willing to do that at this point

[Cap'n Refsmmat]

Your analysis is incorrect when a force changes straight line motion (to curvilinear motion) the movement of the object (while it traverses) is toward the center in line with the force being applied.

No, the object does not move towards the center. It is rotating in a circle and stays at a fixed radius. It neither moves closer nor moves farther away.

 

Work is being performed to cause it to go curvilinear

No, and I'd like to see you provide a source (university, textbook, whatever) that supports this idea. Can you find any reputable source that demonstrates that work is performed in uniform circular motion?

 

If we were to believe your sources we'd have to give up on Newton's Universal Law of Gravitation - and I don't think anyone is willing to do that at this point

Why would we give up on gravitation? I don't see how this discussion is relevant to it. The lack of work doesn't change Newton's law of gravitation.

[gentleman-farmer]

Cap

No, the object does not move towards the center. It is rotating in a circle and stays at a fixed radius. It neither moves closer nor moves farther away.

 

Then there is no tension in the string

[Cap'n Refsmmat]

Then there is no tension in the string

 

Yes, there is. Otherwise, wouldn't the ball at the end just go in a straight line? The string's got to pull on the ball to keep it constrained to a circle, so there's tension.

 

The string's a fixed length, though. It's not getting shorter or something.

[gentleman-farmer]

but if you let go of the sting where will the ball go? Oh - out tangent to the circle?

 

So ya gotta do work toward the center to keep it circular

Edited July 16, 2010 by gentleman-farmer

[Cap'n Refsmmat]

but if you let go of the sting where will the ball go? Oh - out tangent to the circle?

 

So ya gotta do work toward the center to keep it circular

 

No work is involved. Only force. That's the point I've been making over and over and over again.

 

No work is required to keep an object in uniform circular motion. There is an applied force but it does no work.

 

You can look this up in any basic physics textbook that covers circular motion.

[gentleman-farmer]

This discussion is turning foolish so I'm bowing out. As the point is well established if there is tension in the string - than the ball is not going the right way and the string corrects it. To do that there must be work

 

The main point is that if a force exists there must be work first. No way can a scale or transducer measure force without deflection - there'd be nothing to measure

[Bignose]

The definition of work is very clear cut. You may think that with a force there HAS to be work, but that doesn't coincide with the definition that every single physics text uses. The colloquial use of the word is significantly different than the physics use of the word.

 

The physics definition is very clear. And, if a force is perpendicular to the direction of motion, the dot product between 2 perpendicular vectors is zero, by definition.

 

That doesn't mean that nothing is going on. It just means that no work, according to the physics definition is going on.

 

And, as this is a science forum, the physics definition of work is the one that will be used.

[Cap'n Refsmmat]

Indeed. To get the ball going the right way, there only must be force, not work, because not every force results in work. It's part of the definition of work.

[gentleman-farmer]

Cap

not every force results in work. It's part of the definition of work.

 

The definition of work is force through a distance - no mention of not every force results in work.

 

You made that up - that is why I'm out - it is getting foolish

[Cap'n Refsmmat]

The definition of work is [math]W=\mathbf{F}\cdot\mathbf{s}[/math], and any case where [math]\mathbf{F}\perp\mathbf{s}[/math] will result in [math]W=0[/math]. It's part of the definition of work and the dot product.

 

I quoted my textbook already. Perhaps you have one you could refer to?

[Mr Skeptic]

This discussion is turning foolish so I'm bowing out. As the point is well established if there is tension in the string - than the ball is not going the right way and the string corrects it. To do that there must be work

 

The main point is that if a force exists there must be work first. No way can a scale or transducer measure force without deflection - there'd be nothing to measure

 

Just so you know, there's a difference between force, work, and acceleration. For uniform circular motion you have a force (centripetal force), a pseudoforce (centrifugal force), an acceleration (the result of said force acting on the object), and zero work (the displacement parallel to the force is zero). That the work is zero is obvious: the kinetic energy does not change. That there is a force is also obvious: there is an acceleration.

 

Cap

 

The definition of work is force through a distance - no mention of not every force results in work.

 

And we're telling you that when you multiply by zero you get zero. Whatever force you like over zero distance is zero work. It's that simple.

[swansont]

Work by a single force must result in a change in kinetic energy, or else energy is not conserved. If speed is constant, no work could have been done.

 

 

 

 

You made that up

 

Dammit, that's twice you've melted my irony meter.

 

 

!

Moderator Note

Seeing as the originator has promised to abandon the thread, there's no point in discussing this nonsense anymore. Closed.