JamSmith

Jamsmith wisely asked about "the turning effect of forces" and it is quite true that there are several different and distinct mechanical effects associated with turning.

..........

Thanks for detailed information and clarification.

You are correct. Note that the torque is a vector and has a direction perpendicular to that of force and radius vectors.

Translational motion

Newton's law: The force exerted by a body is directly proportional to the rate of change of linear momentum.

Equation: [math]F=ma[/math]

Equations of motion:

1. [math]v=u+at[/math]

2. [math]S=ut+\frac{1}{2}at^2[/math]

3. [math]v^2=u^2+2aS[/math]

 

Rotational motion

Newton's law: The torque on a body is directly proportional to the rate of change of angular momentum.

Equation: [math]\tau = I\alpha[/math]

Equations of motion:

1. [math]\omega_f = \omega_i + \alpha t[/math]

2.[math]\theta = \omega_i t + \frac{1}{2}\alpha t^2[/math]

3. [math]\omega_f^2 = \omega_i^2 +2\alpha \theta [/math]

I was wondering about this solution. Well, thank you Sriman Datt.

I don't like how you formulated this one, as it can be confusing and could confirm common misconceptions about the third law.

More accurately: when object A exerts a moment on object B, object B simultaneously exerts an equal but opposite moment on object A.

Oh ok, I am grateful to you for your more precise discussion.

Moment of force is defined as the turning effect of a force on a rigid body about an axis of rotation.

 

Mathematically the moment of force or torque is a vector, and is equal to the vector cross product of force applied and the perpendicular distance.

[math]\tau = F X r [/math]

 

If the body under application of moments doesn't rotate, then according to the principle of moments, the body is in equilibrium due to the fact that the sum total of clockwise moments is equal to the sum total of anticlockwise moments.

I got your point, I can also follow this right?

Moment (Nm) = Linear force applied at a point (N) X Perpendicular distance between applied point and pivot point (m)

We can then specify the rotational equivalent of Newton's laws:

1. If the moments on a rotating object are balanced then the object remains stationary or at constant angular velocity.

2. If the moments on a rotating object are unbalanced then the object will have an angular acceleration.

3. Moments are balanced by equal and opposite moments

The rotational equivalent of Newton's first law is often called "The principle of moments", which is "if the sum of the clockwise moments is equal to the sum of the anticlockwise moments, the object does not rotate or rotates at constant angular speed".

The concept here is torque, which is the ability to cause rotational acceleration, much like a linear force F causes linear acceleration.

 

T = rF sin(theta), where r is the distance from the pivot and F is the force. Theta is the angle between them; you maximize torque when the two are at right angles

 

But if the beam isn't rotating (balanced), so I have to consider the clockwise moments equal the anti-clockwise moments?

I was solving some online physics question papers.

Can anyone explain, what is Turning Effect of Force?

& also guide me how to solve this:

A uniform beam of 1 m is supported at the 50 cm mark. Given that a weight of 2 N hangs at the 30 cm mark, how far away from the pivot must another weight of 4 N be hung to balance the beam?

There was a long discussion about this recently, so it obviously bothers some beginners.

 

http://www.scienceforums.net/topic/59887-help-in-explaining-formula-of-kinetic-energy/page-2?hl=%2Bkinetic+%2Benergy#entry949583

 

my contribution started at post#39

Thanks. I have found much more clues to solve my doubts.

The formula for kinetic energy is E=[1/2]mv^2 and the formula for momentum is P=mv. I ran into these in physics class long ago and was really bothered by the first formula. How can energy go up as the square of the velocity?

You are right about the pressure. ......

If the bag was sealed at sea level, it then contained a fixed...............

Thank you, both for the guidance.

Think about it.... Up a mountain, is the air hotter? Or is it at a lower pressure?

 

also - what would happen if the air outside the packet was hotter? what would happen if it was at a lower pressure?

Maybe the potato chip bag was sealed at the surface pressure. And the pressure at the top of the mountain is less than surface pressure. So the packet's pressure is higher than the outside pressure and that is the reason why bag inflates like a balloon. I think I am doing right!

I am not sure about what you asked If the air outside hotter, I am just assuming that,

" If the air outside hotter then maybe the bag will slowly inflate like a balloon..!! Because the air has a certain amount of heat, which it carries with it as it rises". or The air inside expands until the pressure inside is the same as that outside.

Some weather balloons explode at height.

And what is the dew point in terms of what it means?

The dew point is the temperature at which air is saturated with water vapor, which is the gaseous state of water.

I have also read that Dew point is closely linked to relative humidity, which is the ratio of the pressure of water vapor in a parcel of air relative to the saturation pressure of water vapor in that same parcel of air at a specific temperature.

Is this satisfying description?

Also think about what is happening when something "fogs up"

 

Have you ever noticed anything fogging up in, say, the bathroom, after taking a shower?

Yes, I have noticed that, and also research on it last night.

I sum up with this:

The fog in the mirror is the moisture of water vapor as it abuts a colder surface. With the cold water, we just cool down the bathroom. So the vapor came from the hot shower, mostly condense and will not reach the mirror.

At the same time, if we do something opposite like vapor comes from heat or hot air will become less likely on the warmer surface.

For air containing a certain amount of moisture, there is a maximum temperature at which the water will Epitomize. This is called the dew point.

Right?

I was recently reading a topic about: What if gravity acted like magnetism?

While I was studying about it, I come across the information which makes me little blunt about basic information.

My questions arise when I read "If you have a positive charge the first thing it does is repeal all the other positive charges around it and attract all the negative charges."

I this happens because of magnetism and the electric force? Is that true that they tend to cancel themselves out? And if its true than what is the reason behind it?

What is the dew point of the air in the house vs the air outside?

I am not sure about it, may be 20° C or below

 

This is a really good way to go about discussing a subject. +1

 

You have presented some basic observations, are they complete?

 

That is

 

Have you considered both plastic and glass lenses?

 

Which side of the lens fogs up, or is it both?

This is an important question that provides a clue to the answer.

 

Have you looked at other glass/plastic objects, for instance window panes?

 

Which side of a window pane fogs up?

I have only tried it on glass.

If Discussiong about Window panes then I observed that when Cooling outside the window fog up inside.

The higher the dew point, the longer it will take for the glasses to clear.

Hi everyone,

I was solving some real time physics question and come across this question.

I have tried to find out the perfect answer but couldn't satisfied with my analysis. Can anyone guide me?

On a cold, dry winter day your glasses fog up when you go indoors after being outside for a while. Why is that?

And if you go back outside with your glasses still fogged up, they quickly clear up. Why is that?

Good answer not sure I could have said it better myself.

 

Lets look at another basic definition.

 

Thanks for the new insight.

 

Gravity is weak at human scales - the usual example is that a fridge has enough attraction to the fridge door to overcome all the mass of the earth; however gravity does not come in two polarities that are opposite and cancel out and as such is always attractive and does not get shielded this means it works on cosmological scales where the other forces tend to be unimportant.

 

So can follow this information right?

The force of gravity is proportional to the size of the object (mass) and your position relative to it (distance from the center of gravity).

Here is the formula:

g=-GM/(r*r),

where, g is the acceleration of gravity,

G is the gravitational constant,

M is the mass of the object and

r is the radius or the distance of the observer from the center of gravity.

I have read somewhre regarding this topic and I come across this information.

Gravity is a weak force. The two objects will have an attraction to each other. But it will be so weak in comparison to other factors and to the attraction to the ground that the attraction to each other will not be noticeable. There is a concerted effort to obscure our understanding of gravity. So we do not see much research on the attraction between small objects.

 

It is good to see you coming back to discuss your topic; connecting entropy and order/disorder can be paradoxical and often leads to surprising or even inappropriate conclusions.

 

As noted here modern science is therefoe veering away from offering this connection.

 

https://en.wikipedia.org/wiki/Entropy_(order_and_disorder)

 

This Wikipedia summary is a fair summary, and says nothing actually incorrect unlike many offerings, although it raises some questions it does not properly answer.

 

The connection really hinges on what you mean by order or disorder.

The definition of entropy is pretty well specified, swansont has provided a statistical definition, and has bender a physical one.

However there is no such convenient definition of either order or disorder. What is meant depends in part on the parameters of interest.

 

You have not discussed these further or indicated you mathematical level but you can't fully consider the question without some mathematics.

 

Going with the statistical approach, since it is in the title, here is a simple introduction.

 

Consider a chessboard : It has 64 squares.

 

Which means there are 64 ways to place a single black pawn on the (otherwise empty) board.

There is no reason to assume any of these positions or arrangements is 'better' than any other so we choose one square and call it 'order'.

If we place the pawn there the arrangement is 'ordered'.

If we place it anywhere else the arrangement is 'disordered'.

There is thus 1 possible arrangement called order but there are 63 possible ways of disorder.

Now we consider a change of arrangement.

If we make a single change to arrangement, i.e. move a disordered pawn to any other square, there are 62 ways of doing this whilst maintaining the disorder and only one way to change to an ordered pawn.

For this system a change is 62 times more likely to result in disorder than order.

 

This is only for one single pawn.

 

Now take all 8 black pawns and consider arrangements of them on the board.

 

You can place the first pawn in one of 64 ways i.e.on any square.

You can place the second pawn in one of 63 ways i.e.on any remaining square.

You can place the third pawn in one of 62 ways i.e.on any remaining square.

and so on.

 

In total this means there are 64 x 63 x 62 x 61 x 60 x 59 x 58 x 57 = 178462987637760 different arrangements.

So what is order now?

Is it perhaps some relationship between the positions of the pawns, say they are all in a straight line?

 

Well there are 2 ways this can be done if they are to remain on the same colour and another 16 ways if the colour does not matter.

 

This simple model can be developed to embody all the important characteristics of Statistical Mechanics which are, in relation to the OP.

 

1) The pawns are not distinguished. Every pawn is equivalent to every other, so any pawn in the order position constitutes order.

 

2) The arrangements are not distinguished so any position can be chosen as order.

 

3) It is changes to the position that offer meaningful properties.

 

4) When talking about the change, only the beginning and end positions are meaningful. No meaning is attached to the positions during the change.

 

Notes what I have called arrangements or positions are called states in Thermodynamics and Statistical Mechanics.

 

I did get the article. It looked nice. The volume part change of entropy is directly related to disorder. See this article

 

http://www.pa.msu.edu/courses/2005spring/phy215/phy215wk4.pdf

 

The temperature entropy relation to number of states are as far as I can see related to amount of energy in the system, the more energy the more entropy. And the higher number of molecules, the more energy and more enntropy. The temperature side of entropy as I have learned it is related to the boltzman distribution which is one of the main theorems in thermodynamics. You could find a derivation in the litterature.Atkins physical chemistry was were I found the most info about it myself.

I got your point. Thanks for co-operation.

It is also possible to rewrite the micro def of entropy S=klnW to the macro def of entropy [math]S=\frac{Q}{T}[/math] both for temperature and volume change. And since S=klnW gives number of combinations that gives the entropy for a given p,V,T it means that the law that max combinations, max entropy (as given in statistical thermodynamic) is most spontaneous can be derived mathematically to the macro version as well. The macro version and calculations are often used in introduction courses in thermodynamics without this derivation. So to me as I look back it seems a bit backward when you take the course that they dont enlighten us with a derivation of entropy from micro to macro mathematically first.

Thanks for the concept. But if we think of entropy as "disorder", I come across so many articles. I Found this one short and very clear. http://webs.morningside.edu/slaven/Physics/entropy/entropy7.html

Is it relavent blog to follow?

Entropy is a concept from thermodynamics, which doesn't study the behaviour of individual particles, but (very) large quantities, hence "statistical" mechanics. Examples of statistical properties are pressure and temperature, but also entropy and its complement: enthalpy.

 

In short: the useful energy in a gas can be split in two parts: entropy and enthalpy. Simply put:

- enthalpy is useful energy, which can e.g. be converted to electricity in a turbine

- entropy is the waste heat that cannot be recovered

 

The relation between entropy and disorder becomes clear in the second law of thermodynamics:

[math]\frac{dS}{dt} \geqslant 0[/math]

with S the entropy. In words it says: the entropy of a closed system can only increase over time. The typical example is a vase that falls from the table and shatters, while the shards will never accidentally become a complete vase again.

Another example is two containers with air at different pressure: there is useful energy in them, because the pressure difference can be used for mechanical work. When they are connected, the gas will mix and the result will be two containers on the same pressure. While the total energy content did not change (conservation of energy), the result is useless. Something similar will happen with different temperatures and mixtures of fluids: when there is a difference (order), this can be used to produce work, when it is all homogeneous (disorder), it cannot be converted to work.

The transition from order to disorder happens naturally (e.g. objects in contact will evolve to the same temperature; or milk will spread through coffee), the transition from disorder to order can only happen through outside influence (and requires energy).

 

EDIT: a consequence of this is that a combustion engine has a maximum possible efficiency, generally about 40% (Carnot efficiency). This is not a practical limit of how we make engines, but a theoretical limit dictated by thermodynamics.

Thank you Bender.

So finally this follows the rule : A natural process (reaction) causes an increase of entropy or entropy increase is the driving force for natural reactions.

If there is an err please correct me.

I had discussion on statistical mechanics with my friend last night. I come to know, The relationship between entropy and disorder is studied in a discipline called statistical mechanics.

I have no cleare Idea about , How is Entropy Related to Disorder?

Can anyone help me to clear my mind.?