[Sutripta]

Originally Posted by mallumowgli

Err...aren't you guys both saying the same thing??

No. Cause and effect.

Also I think in the above problem since the force applied and the movement is linear, the equation of torque will not be apt.

Almost similar concepts (and misconceptions).

@Sutripta - I think you want the string to be tied at the other end just because you want the force at t=0 be the maximum possible in case 1. Essentially the above described 2 cases are the same, except that t=0 are at two different points

Irrelevant. Just specify at t=0 a force F is applied.

The interesting thing to note is that at t=0, the two cases are identical. Yet it is assumed that in the first case the block will remain stationary (tension in string = 0) whereas in the second case it will move (accelerate). Something has to give! Should have concentrated on this (and was the reason for giving these particular examples) but got diverted.

Regards
Sutripta

[amit_purohit20]

We are in agreement that if somehow there is a force/ torque on a body the body will accelerate. This was never a disagreement, but let me delve on the other topics or example in discussion ie how torque will be there once clamps are released.
But I would not like you to deviate from the course and the momentum you have built on the example. Please go on. You are making me think hard

[Sutripta]

^^^
The problem statement did say constant force! Once again strings, weights, and pulleys/ sheaves to our rescue if you want a simple visualisation.

So once again, what can we say about our system (ie tension in the string, description of the movement, if any, of the block).

Regards
Sutripta

[amit_purohit20]

Instead of asking questions I would like you to explain why the Torque should not fall down inspite of the clamps released.
By answering your questions I feel am getting diverted from the topic or not able to get to our main point as to why torque should not fall down once the clamps are released.

[amit_purohit20]

Originally Posted by Sutripta

^^^
The problem statement did say constant force! Once again strings, weights, and pulleys/ sheaves to our rescue if you want a simple visualisation.

So once again, what can we say about our system (ie tension in the string, description of the movement, if any, of the block).

Regards
Sutripta

Sutripta,

I have gone through the last 3 pages again and I got your point. You want to say that once the clamps are released the full torque will be applied to the wheel.This will happen because its a constant torque source also using the first and second Newtons laws. I agree to this. I got your point from the metal cube example.

Now once full torque from source is applied to wheel. The wheel will start accelerating and stabilise at a constant acceleration value using eq. T=I*alpha (acceleration)

The velocity will go on increasing but as you said if the load on the system increases monotonically and is a function of velocity.

Over a period of time the force/torque applied will be equal to the drag forces and the wheel will stabilize at one particular rpm where the drag force/torque equals the applied torque.
Great I get it upto here.

As we considered a theoretical constant torque source the rpms were limited only due to drag forces and not because of torque source limitations. Also we assumed that the torque source can apply the constant torque at all rpms till infinity.

1) But what happens in a real engine? The wheel rpms are limited because of engine limitations or because of drag torque equalling applied torque?

2) Can we also try to explain the above example in terms of Power?
P=K*N*T

As soon as clamps are released the torque will remain constant and cause acceleration (rpms) so Power will go on increasing.

When the rpms stabilize due to the drag forces the Power will be the maximum right?

Also now when I look at the bicycle chain snapping example. I get a jerk because I am human and cannot apply constant torque to the axle like a constant torque source. If my legs were a constant torque source which can rotate upto infinity I would have not felt the jerk which I get due to chain snatching as before and after torque values would have remained same.
So because I am not a constant torque source I cannot apply/maintain the same torque. If I cannot apply/maintain the constant torque source the response by the wheel/axle would be different.

Considering our previous example if instead of constant torque source we used a engine (which is not a constant torque source) what would happen to the wheel before and after removal of clamps (Forget about engine stalling here)

I feel the engine (not a constant torque source) will apply max torque at the clamps but as soon as the clamps are released there will be reduction in resistance. As the engine generates only that much torque which is resisted the torque generated by engine will gradually fall down.

This fall in torque avoids the very high rise of wheel rpms which were expected in the constant torque source case. Also to add to it the drag from the wheel and the engine itself will help more in controlling the wheel rpms to shoot at a very high level.

[Sutripta]

Originally Posted by amit_purohit20

Sutripta,

I have gone through the last 3 pages again and I got your point. You want to say that once the clamps are released the full torque will be applied to the wheel.This will happen because its a constant torque source also using the first and second Newtons laws. I agree to this. I got your point from the metal cube example.

Great!

I strongly believe that one must understand and appreciate the theoretical foundations before going on to something more complex. Fail to do so, and one will emulate a headless chicken in a maze! In our case it would be understanding the effect of (simple) forces on masses in an idealised environment from which all complexities have been removed. Once we are at this stage, we can add complexities to our ideal model, one variable at a time, and work out its effect. Even in this we (normally) make simplifying assumptions about the complexities/ variables we are going to introduce. (Whenever we make assumptions/ simplifications, we must be very careful to see to it that we remove that which is superfluous to the current problem. This choice of what to retain, and what to remove, indeed on how to model, is not trivial.)

It is because of this belief that instead of going into endless discussions, decided to go to the very basics. Going forward (if there is any need. I feel most people have understood the topic of this thread perfectly by now) I would still like to retain our simple model of a block of metal pulled by a string. Later on, it can very simply be extended to the torque/ wheel scenario.

There are a few assumptions in our model
The mass of the block remains constant
The (driving) force remains constant
All other forces are nonexistent (remember frictionless horizontal plane) Should have added no drag forces too!
The string is massless, inextensible, etc.

We can now make changes to these assumptions to see what effect it will have.

So what are the changes you would like to introduce?

Regards
Sutripta

The metric for engines or motors is KW/ HP. How are jets and rockets specified?

[Jeroen]

Originally Posted by Sutripta

I strongly believe that one must understand and appreciate the theoretical foundations before going on to something more complex. Fail to do so, and one will emulate a headless chicken in a maze!

The metric for engines or motors is KW/ HP. How are jets and rockets specified?

Amen to that!

Jet and or rocket engines are specified, typically, by thrust ratings. So in pound (lbs) or for the more metric orientated amongst us, Newton meter (Nm).

Typically you don't specify them in KW/HP, because its simply not meaningfull. What moves the plane / rocket is thrust. There is a term called propulsive power which is sometimes used. In essence propulsive power is proportional with speed. It increases, linear, as the speed increases.

Jeroen

[amit_purohit20]

Originally Posted by Jeroen

Amen to that!

Jet and or rocket engines are specified, typically, by thrust ratings. So in pound (lbs) or for the more metric orientated amongst us, Newton meter (Nm).


Jeroen

Thrust should in Newtons and not Newton meter right?

A big thanks to Sutripta for clearing lot of queries and doubts. I was busy a lot so couldnot be regular on this thread.

[Jeroen]

Originally Posted by amit_purohit20

Thrust should in Newtons and not Newton meter right?

yes, you are absolutely correct, my mistake, well spotted

So, just to add, to calculate the propulsive power of a jet, you take the trust (N) and multiply with speed (m/s) and you get power (Nm/s) i.e. watt.

Jeroen

[Sutripta]

Originally Posted by Jeroen

Jet and or rocket engines are specified, typically, by thrust ratings.

Thanks!
But the purpose of the question was to get (at least some) people to think. As to why jets/ rockets are not/ should not be/ cannot be specified in terms HP. (Power has been referred to a couple of times in this thread).

In my experience, 'solved' questions and answers is just an euphemism for a cheat sheet for a dumbed down generation. This thread is a very good example of the misconceptions which arise when one does not think through from the very basics. Just giving answers is just not going to work. (At that point it is just the opinion of X vs Y). But if they think through on their own ...

@ Amit, really glad if I could be of any help.

Regards
Sutripta

[Jeroen]

Originally Posted by Sutripta

Thanks!
But the purpose of the question was to get (at least some) people to think. As to why jets/ rockets are not/ should not be/ cannot be specified in terms HP.

Got you! Remind me not to answer your questions correctly in future!

Jeroen

[Sutripta]

Originally Posted by Jeroen

Got you! Remind me not to answer your questions correctly in future!

Jeroen

Solutions by PM. Questions on the board! (The mods are not going to be happy!)
(Wrong answers are a load of fun though!)

Come on. Out of the blue, why else would I suddenly ask that question?

Regards
Sutripta

[DirtyDan]

Originally Posted by Sutripta

... Just giving answers is just not going to work. (At that point it is just the opinion of X vs Y). But if they think through on their own ...

I just got back from a 1000K trip from Dharamsala, H.P. to Western Nepal and back. I can truly testify that the roads of Northern India are NOT frictionless planes. And, the IFS of my Thar did not save my backside from repeated collisions with unsympathetic stock seats.

There are times in this thread when what constitutes a "Proof" has been a question in my mind. Ordinarily science tests theories for logical consistency, mathematical soundness....and finally by publically verifiable and repeatable experiment. We don't have that here in this thread. We only have logic, analogy and the power of language to convince and persuade. It's amazing to me that we have come this far here.

I hope the moderators will loosen their surly bonds and let us delve into stuff that may at first seem not relevant....in a thread whose very relevance to 4x4 and off-roading may seem to be a bit nebulous. At any rate, I would like to say that my off-road driving has improved 34.567% since the inception of this thread. The families of several field mice and a couple of mongoose wish to thank the contributors to this thread for making me a better and more careful driver.

Who knows, we may stumble on cold fusion from discarded axle grease and basmati rice right here and this thread will be famous...or infamous.

What's next on the docet?

[Sutripta]

Originally Posted by DirtyDan

Ordinarily science tests theories for logical consistency, mathematical soundness....and finally by publically verifiable and repeatable experiment.

Peer review is the word (or two) you're searching for!

Regards
Sutripta

[DirtyDan]

Originally Posted by Sutripta

Peer review is the word (or two) you're searching for!

Regards
Sutripta

Well, I was not really searching for words but, yeah, that's what we have got. I guess we can make do.