Misconceptions about Forces

[gentleman-farmer]

It was said on another thread that ( snip )

allow(s) the possibility of exerting a force that does no work

 

It was also stated that

Just because forces are involved does not mean work is being done

 

gf) The purpose of this thread is to bring people's attention to the fact that both of those statements are false. They are false because it is impossible to have a force manifest itself without having encountered resistance. Without resistance force does not exist.

 

With resistance there is deflection - with deflection there is distance traveled - with distance traveled there is work - hence no force can exist but what work has been already been performed

 

The same is true of fluid systems - pressure cannot exist but what there is resistance down stream

 

gf

/

[D H]

It's not a good idea to try to start a thread on a closed topic.

 

I've asked you many times for the definition of work. You never did reply. Here it is:

 

[math]W\equiv\int_c\vec F \cdot d\vec l = \int_c \vec F \cdot \vec v dt[/math]

 

In order for a force to do work the force must have a component parallel to (or antiparallel to) the motion. The component of force normal to the motion does zero work.

[gentleman-farmer]

I wouldn't know why you'd ask me for a definition of work - it's in many high school science books and in all physics books. It is easier recognized simply as force through a distance (f x d) the units in the English system are foot pounds (same as energy)

 

The important thing is that force does not exist but what work has been achieved first - so your statement is false

/

Just because forces are involved does not mean work is being done

 

And if you asked me for a definition of work - it had to be on another thread!

 

gf

/

[swansont]

I wouldn't know why you'd ask me for a definition of work - it's in many high school science books and in all physics books. It is easier recognized simply as force through a distance (f x d) the units in the English system are foot pounds (same as energy)

 

Maybe it's because you keep repeating statements that contradict the definition.

 

 

 

 

Work is also equal to the change in kinetic energy. If an object moves in a circle at constant speed, the KE is unchanging. How do you reconcile that?

[darkenlighten]

Actually just because there is a force does not mean that force is doing work. The best example is the magnetic field, which NEVER does work, but yet there is a force involved.

[gentleman-farmer]

darkenlighten

Actually just because there is a force does not mean that force is doing work. The best example is the magnetic field, which NEVER does work, but yet there is a force involved.

 

gf) Not true - if a force exists work must be performed first -- as for magnetic fields - solenoids are a good example

 

That aside we're not talking about magnetic fields - I was as specific as I could be - we're talking about work meaning force through a distance (f x d) the units in the English system are foot pounds (same as energy) -

[Cap'n Refsmmat]

So suppose I have a one-kilogram box. I put it on the ground. I apply a force of ten Newtons to one side, and a force of ten Newtons to the opposite side, such that the forces oppose each other equally.

 

No work is done, because the box does not move.

 

Does that mean there is no force? You say work must be performed before a force can exist.

[gentleman-farmer]

Cap'n Refsmmat

So suppose I have a one-kilogram box. I put it on the ground. I apply a force of ten Newtons to one side, and a force of ten Newtons to the opposite side, such that the forces oppose each other equally.

 

Cap'n Refsmmat

No work is done, because the box does not move.

 

Does that mean there is no force?

 

gf) No! It means you have not analyzed the problem correctly.

 

The law you are looking for is Newton's 3rd Law: For every action there is an equal and opposite reaction

 

Each side of the box is going to respond elastically with application of the forces. The resultant deflection times the average force will be the work performed

 

That the box itself doesn't moved is irrelevant - the work was performed in deflecting the sides

 

gf

\

[Cap'n Refsmmat]

gf) No! It means you have not analyzed the problem correctly.

 

The law you are looking for is Newton's 3rd Law: For every action there is an equal and opposite reaction

 

Each side of the box is going to respond elastically with application of the forces. The resultant deflection times the average force will be the work performed

 

That the box itself doesn't moved is irrelevant - the work was performed in deflecting the sides

 

gf

I don't quite see your point. The deflection is finite; after a while (perhaps a second or two), the deflection will have reached its maximum and the sides will be fully opposing my applied forces. After that point, there will be no further deflection.

 

Imagine pressing on a cardboard box. The cardboard will bend slightly, but unless you increase the force applied, it will only bend so far.

 

But after that point, I'm still applying a force. I'm still pressing on the sides, despite the fact that they are not moving. But there's no work.

[gentleman-farmer]

Cap'n Refsmmat

I don't quite see your point. The deflection is finite; after a while (perhaps a second or two), the deflection will have reached its maximum and the sides will be fully opposing my applied forces. After that point, there will be no further deflection.

 

gf) Exactly right - but don't think work hasn't been performed. Let's pretend the box's sides were highly elastic and you let go of what ever you used to apply the force. The sides would rebound and put whatever you were holding - right into your pocket

 

The concept is not unlike a simple bow

 

gf

/

[Cap'n Refsmmat]

gf) Exactly right - but don't think work hasn't been performed. Let's pretend the box's sides were highly elastic and you let go of what ever you used to apply the force. The sides would rebound and put whatever you were holding - right into your pocket

 

The concept is not unlike a simple bow

 

gf

 

Sure. But once I deflect the sides as far as they'll deflect with my ten Newtons, is there no longer a force? The sides aren't moving, so there's no longer work being done, but I am still applying a force, aren't I?

[Mr Skeptic]

gf) The purpose of this thread is to bring people's attention to the fact that both of those statements are false. They are false because it is impossible to have a force manifest itself without having encountered resistance. Without resistance force does not exist.

 

That you think it is false, simply shows that you lack education and common sense. It takes no genius to realize that a weight sitting on a table not moving is not doing work. As for the nonsense about you having taught college physics, there is no way you could have taught college physics without knowing what work is. So this makes you a liar as well. There's a reason they teach the simplified equation for work in high school: most kids don't know calculus. However they do also teach about the distance component having to be parallel to the force, in high school. And the "everyone disagrees with me, I must be so much smarter than them" attitude shows arrogance as well.

 

Anyhow, here, please go learn something about forces before trying to teach others about them. It would save everyone so much time.

http://en.wikipedia.org/wiki/Work_(physics)

 

In particular this bit:

However, if the force and the displacement act perpendicular to each other, zero work is done by the force:[4]

 

W = 0

[gentleman-farmer]

Cap'n Refsmmat

But once I deflect the sides as far as they'll deflect with my ten Newtons, is there no longer a force? The sides aren't moving, so there's no longer work being done, but I am still applying a force, aren't I?

 

gf) You can answer that from our own experience - let me hand you a 175 pound (pull) bow. Now insert an arrow and pull it back - now hold it while I get some coffee

 

The deflection does not go away - it sits there waiting for you to make a mistake

 

gf

/

[Mr Skeptic]

Also I note that in this thread you were disputing the possibility that a force that does no work exists, not the possibility that a force that has never done any work exists. Don't move the goalposts.

[Cap'n Refsmmat]

Cap'n Refsmmat

 

gf) You can answer that from our own experience - let me hand you a 175 pound (pull) bow. Now insert an arrow and pull it back - now hold it while I get some coffee

 

The deflection does not go away - it sits there waiting for you to make a mistake

 

gf

/

 

I know. While I deflect the bow, I am doing work, because I am applying a force to move the bowstring over a distance.

 

Once I have deflected the bow to where I want it, I am applying a force. However, I am not applying it over a distance. It's already there. No additional work is being done. But there's still a force! So why is work necessary for there to be a force?

[Mr Skeptic]

Cap'n Refsmmat

 

gf) You can answer that from our own experience - let me hand you a 175 pound (pull) bow. Now insert an arrow and pull it back - now hold it while I get some coffee

 

The deflection does not go away - it sits there waiting for you to make a mistake

 

gf

/

 

For a human to hold a bow is work, which again is the integral of force times distance parallel to the force. No work is being done on nor by the bow; the work is in moving actin and myosin molecules. (In fact for a human to do "nothing" is also work.) Try this: put a stick holding the bow bent. How much work is the stick doing? As for that cup of coffee, try not eating and not doing work for a couple months, and I'll tell you again about how humans actually do internal work despite not doing any external work.

[gentleman-farmer]

Cap'n Refsmmat

I know. While I deflect the bow, I am doing work, because I am applying a force to move the bowstring over a distance.

 

Cap'n Refsmmat

Once I have deflected the bow to where I want it, I am applying a force. However, I am not applying it over a distance. It's already there. No additional work is being done. But there's still a force! So why is work necessary for there to be a force?

 

gf) No! You are not just moving the bowstring over a distance - you are deflecting the material fibers of the bow - (the string is just a tool)

 

At that point the work is complete and the initial force has already manifested itself in having performed work. No force existed before the bow started to deflect - that is the point of interest - what you do thereafter is another story

[Cap'n Refsmmat]

gf) No! You are not just moving the bowstring over a distance - you are deflecting the material fibers of the bow - (the string is just a tool)

 

At that point the work is complete and the initial force has already manifested itself in having performed work. No force existed before the bow started to deflect - that is the point of interest - what you do thereafter is another story

 

That's all fine. But what about when I hold the bowstring in the same position? At that point, the work is complete, but I'm still applying a force to hold the bowstring that way, aren't I?

 

For a human to hold a bow is work, which again is the integral of force times distance parallel to the force. No work is being done on nor by the bow; the work is in moving actin and myosin molecules. (In fact for a human to do "nothing" is also work.) Try this: put a stick holding the bow bent. How much work is the stick doing? As for that cup of coffee, try not eating and not doing work for a couple months, and I'll tell you again about how humans actually do internal work despite not doing any external work.

 

But my force on the bow is not doing any work, and that's what matters. You're making this more complicated for the sake of semantics.

[gentleman-farmer]

Cap'n Refsmmat

That's all fine. But what about when I hold the bowstring in the same position? At that point, the work is complete, but I'm still applying a force to hold the bowstring that way, aren't I?

 

gf) From the point of view of the bow - the work is complete - but the work is stored in the deflected fibers of the bow. They are applying a force to your poor tiring body.

 

Let's change the scene - we'll follow the path of a projectile as it leaves the barrel - the only force that exists is what it takes to overcome aerodynamic drag.

 

Then the projectile hits a board set in its way - now a force materializes as it goes through the board (time rate of change of momentum) - once out no force exists

 

Then it hits a house and forces materialize as the projectile hits the respective walls (and again - time rate of change of momentum)

 

In between the walls - no force exists

 

The point being that until a perspective event meets resistance no force exists

 

gf

/

[Cap'n Refsmmat]

gf) From the point of view of the bow - the work is complete - but the work is stored in the deflected fibers of the bow. They are applying a force to your poor tiring body.

And I'm still applying a force to the bow so it doesn't fire, right? I still have to hold the bowstring back. But the work on the bow is complete.

 

Thus, a force exists without causing any work.

 

Now, consider a different situation. I have a nail, a string, and an object. The string ties the object and nail together. The object is rotating around the nail at a constant velocity, attached by the string -- much as a tetherball is attached to the pole.

 

The object is rotating around at a constant angular velocity. The string is under tension to keep the object going in a circle. It has to apply a force on the object to keep it rotating.

 

But the object, in the absence of friction, continues rotating at a constant angular velocity.

 

Is the string doing work because of its force?

[gentleman-farmer]

Cap'n I've explained it every which way but loose - I'll offer this - extend our arm out as far as it'll go. Nothing happens (no force - no work) now your hand touches a post. The post moves - you feel a force (that didn't exist a moment ago) there was work performed on the post as it tipped - and you felt the force increase as the post moved

 

I hope this helps

 

gf

[Cap'n Refsmmat]

No, you haven't explained it. From what I've gathered, you're arguing that the string pulling the object in the circle is performing work. Is that right?

[gentleman-farmer]

Cap'n

the string pulling the object in the circle is performing work. Is that right?

 

gf) Ya got me there Cap - I haven't the foggiest idea what you're talking about - but if that's what ya wanna believe - than go for it

 

As for me - I know that unless an event touches something and that something starts to deflect - no force registers. It's like ya can't get kilt by a tree until it falls on ya

[Cap'n Refsmmat]

gf) Ya got me there Cap - I haven't the foggiest idea what you're talking about - but if that's what ya wanna believe - than go for it

 

As for me - I know that unless an event touches something and that something starts to deflect - no force registers. It's like ya can't get kilt by a tree until it falls on ya

 

I have a ball on a string that I'm spinning around. The string has a tension and pulls the ball around in the circle. The ball is traveling at a constant angular velocity.

 

Got that?

 

Now, is the string doing any work on the ball?

[gentleman-farmer]

Cap asks

is the string doing any work on the ball?

 

gf) The ball is wanting to go straight line in accordance with Newton's 1st Law: a body in motion will follow a straight line unless acted upon by an external force

 

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 -