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Physics Study Guide/Force

=Force= A net force on a body causes a body to accelerate. The amount of that acceleration depends on the body's inertia (or its tendency to resist changes in motion), which is measured as its mass. When Isaac Newton formulated Newtonian mechanics, he discovered three fundamental laws of motion.

Later, Albert Einstein proved that these laws are just a convenient approximation. These laws, however, greatly simplify calculations and are used when studying objects at velocities that are small compared with the speed of light.

Friction
It is the force that opposes relative motion or tendency of relative motion between two surfaces in contact represented by f. When two surfaces move relative to each other or they have a tendency to move relative to each other, at the point (or surface) of contact, there appears a force which opposes this relative motion or tendency of relative motion between two surfaces in contact. It acts on both the surfaces in contact with equal magnitude and opposite directions (Newton's 3rd law). Friction force tries to stop relative motion between two surfaces in contact, if it is there, and when two surfaces in contact are at rest relative to each other, the friction force tries to maintain this relative rest. Friction force can assume the magnitude (below a certain maximum magnitude called limiting static friction) required to maintain relative rest between two surfaces in contact. Because of this friction force is called a self adjusting force. Earlier, it was believed that friction was caused due to the roughness of the two surfaces in contact with each other. However, modern theory stipulates that the cause of friction is the Coulombic force between the atoms present in the surface of the regions in contact with each other.

Formula: Limiting Friction = (Friction Coefficient)(Normal reaction)

Static Friction = the friction force that keeps an object at relative rest.

Kinetic Friction = sliding friction

Newton's First Law of Motion
(The Law of Inertia)

 * A static object with no net force acting on it remains at rest or if in movement it will maintain a constant velocity

This means, essentially, that acceleration does not occur without the presence of a force. The object tends to maintain its state of motion. If it is at rest, it remains at rest and if it is moving with a velocity then it keeps moving with the same velocity. This tendency of the object to maintain its state of motion is greater for larger mass. The "mass" is, therefore, a measure of the inertia of the object.

In a state of equilibrium, where the object is at rest or proceeding at a constant velocity, the net force in every direction must be equal to 0.

At a constant velocity (including zero velocity), the sum of forces is 0. If the sum of forces does not equal zero, the object will accelerate (change velocity over time).

It is important to note, that this law is applicable only in non-accelerated coordinate systems. It is so, because the perception of force in accelerated systems are different. A body under balanced force system in one frame of reference, for example a person standing in an accelerating lift, is acted upon by a net force in the earth's frame of reference.

Inertia is the tendency of an object to maintain its velocity i.e. to resist acceleration.
 * Inertia is not a force.
 * Inertia varies directly with mass.

Newton's Second Law of Motion

 * The time rate of change in momentum is proportional to the applied force and takes place in the direction of the force.

 * 'The acceleration of an object is proportional to the force acting upon it.

These two statements mean the same thing, and is represented in the following basic form (the system of measurement is chosen such that constant of proportionality is 1) :

The product of mass and velocity i.e. mv is called the momentum. The net force on a particle is thus equal to rate change of momentum of the particle with time. Generally mass of the object under consideration is constant and thus can be taken out of the derivative.

Force is equal to mass times acceleration. This version of Newton's Second Law of Motion assumes that the mass of the body does not change with time, and as such, does not represent a general mathematical form of the Law. Consequently, this equation cannot, for example, be applied to the motion of a rocket, which loses its mass (the lost mass is ejected at the rear of the rocket) with the passage of time.

An example: If we want to find out the downward force of gravity on an object on Earth, we can use the following formula:

Hence, if we replace m with whatever mass is appropriate, and multiply it by 9.806 65 m/s2, it will give the force in newtons that the earth's gravity has on the object in question (in other words, the body's weight).

Newton's Third Law of Motion
This means that for every force applied on a body A by a body B, body B receives an equal force in the exact opposite direction. This is because forces can only be applied by a body on another body. It is important to note here that the pair of forces act on two different bodies, affecting their state of motion. This is to emphasize that pair of equal forces do not cancel out.
 * Forces occur in pairs equal in magnitude and opposite in direction

There are no spontaneous forces.

It is very important to note that the forces in a "Newton 3 pair", described above, can never act on the same body. One acts on A, the other on B. A common error is to imagine that the force of gravity on a stationary object and the "contact force" upwards of the table supporting the object are equal by Newton's third law. This is not true. They may be equal - but because of the second law (their sum must be zero because the object is not accelerating), not because of the third.

The "Newton 3 pair" of the force of gravity (= earth's pull) on the object is the force of the object attracting the earth, pulling it upwards. The "Newton 3 pair" of the table pushing it up is that it, in its turn, pushes the table down.

Equations
To find Displacement

To find Final Velocity

To find Final Velocity

To find Force when mass is changing

To find Force when mass is a constant

دليل دراسة الفيزياء/القوى

title: Physics Study Guide/Force


text:
User:Tul54161/Evaluate an Article

Evaluate an article
This is where you will complete your article evaluation. Please use the template below to evaluate your selected article.

 * Name of article: Club sandwich
 * Briefly describe why you have chosen this article to evaluate: I have chosen the Club Sandwich article to evaluate because when looking at the list of food/drinks articles I was not familiar with a lot of the topics that were written about.

Lead

 * Guiding questions

The lead includes what the topic is and the two names it goes by (club sandwich/clubhouse sandwich). It states all the ingredients for the sandwich and how it is served in modern day.

 * Does the Lead include an introductory sentence that concisely and clearly describes the article's topic?
 * Does the Lead include a brief description of the article's major sections?
 * Does the Lead include information that is not present in the article?
 * Is the Lead concise or is it overly detailed?

Content

 * Guiding questions

In the ingredients section of the article, Burger King is mentioned and how the fast food chain got criticism for how unhealthy their club sandwich was back in 2000. This section of the article is not related to ingredients and could be moved to "history" of club sandwiches.

 * Is the article's content relevant to the topic?
 * Is the content up-to-date?
 * Is there content that is missing or content that does not belong?

Tone and Balance

 * Guiding questions

The article seems unclear about the origin of the club sandwich and what restaurant and chef in New York started the phenomenon.

 * Is the article neutral?
 * Are there any claims that appear heavily biased toward a particular position?
 * Are there viewpoints that are overrepresented, or underrepresented?
 * Does the article attempt to persuade the reader in favor of one position or away from another?

Sources and References

 * Guiding questions

The references include sources that are from random blog posts that some may consider unreliable.

 * Are all facts in the article backed up by a reliable secondary source of information?
 * Are the sources thorough - i.e. Do they reflect the available literature on the topic?
 * Are the sources current?
 * Check a few links. Do they work?

Organization

 * Guiding questions

The article is well written, it includes every ingredient that goes into a club sandwich, including vegan club sandwiches.

 * Is the article well-written - i.e. Is it concise, clear, and easy to read?
 * Does the article have any grammatical or spelling errors?
 * Is the article well-organized - i.e. broken down into sections that reflect the major points of the topic?

Images and Media

 * Guiding questions

When hovering your mouse on the links to the ingredients a picture is shown for each of them. There is also a picture of a club sandwich in the beginning of the article.

 * Does the article include images that enhance understanding of the topic?
 * Are images well-captioned?
 * Do all images adhere to Wikipedia's copyright regulations?
 * Are the images laid out in a visually appealing way?

Checking the talk page

 * Guiding questions

The talks page top results are people disproving who created the club sandwich and commenting on how terrible the picture of the club sandwich is. Other comments include titles like "Huh?" and "Sub?", stating that there are no sources connected to Danny Mears, who is the only person mentioned in the article.

 * What kinds of conversations, if any, are going on behind the scenes about how to represent this topic?
 * How is the article rated? Is it a part of any WikiProjects?
 * How does the way Wikipedia discusses this topic differ from the way we've talked about it in class?

Overall impressions

 * Guiding questions

The article could be improved with the history of the club sandwich. It seems like the person who added the information was not completely sure on the history ad did not sight creditable sources. It could be improved by using sources that does not lead to random blog sights that are no longer in use.

 * What is the article's overall status?
 * What are the article's strengths?
 * How can the article be improved?
 * How would you assess the article's completeness - i.e. Is the article well-developed? Is it underdeveloped or poorly developed?

Optional activity

 * Choose at least 1 question relevant to the article you're evaluating and leave your evaluation on the article's Talk page. Be sure to sign your feedback

with four tildes — ~

 * Link to feedback:

source: wikimedia

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: User:Tul54161/Evaluate an Article


text:
countrie

Noun

 * 1) * 1903 Aucassin and Nicolette (translated from the Old French by Francis William Bourdillon), p.67:
 * All the kingdom and countrie / Held in great tranquillity.
 * 1) * 1903 Aucassin and Nicolette (translated from the Old French by Francis William Bourdillon), p.67:
 * All the kingdom and countrie / Held in great tranquillity.

title: countrie


text:
C Programming/Memory management

In C, you have already considered creating variables for use in the program. You have created some arrays for use, but you may have already noticed some limitations:
 * the size of the array must be known beforehand
 * the size of the array cannot be changed in the duration of your program

Dynamic memory allocation in C is a way of circumventing these problems.

The function
The standard C function  is the means of implementing dynamic memory allocation. It is defined in stdlib.h or malloc.h, depending on what operating system you may be using. Malloc.h contains only the definitions for the memory allocation functions and not the rest of the other functions defined in stdlib.h. Usually you will not need to be so specific in your program, and if both are supported, you should use , since that is ANSI C, and what we will use here.

The corresponding call to release allocated memory back to the operating system is.

When dynamically allocated memory is no longer needed,  should be called to release it back to the memory pool. Overwriting a pointer that points to dynamically allocated memory can result in that data becoming inaccessible. If this happens frequently, eventually the operating system will no longer be able to allocate more memory for the process. Once the process exits, the operating system is able to free all dynamically allocated memory associated with the process.

Let's look at how dynamic memory allocation can be used for arrays.

Normally when we wish to create an array we use a declaration such as

Recall  can be considered a pointer which we use as an array. We specify the length of this array is 10 s. After , nine other integers have space to be stored consecutively.

Sometimes it is not known at the time the program is written how much memory will be needed for some data; for example, when it depends upon user input. In this case we would want to dynamically allocate required memory after the program has started executing. To do this we only need to declare a pointer, and invoke  when we wish to make space for the  elements in our array, or, we can tell   to make space when we first initialize the array. Either way is acceptable and useful.

We also need to know how much an int takes up in memory in order to make room for it; fortunately this is not difficult, we can use C's builtin  operator. For example, if  yields 4, then one   takes up 4 bytes. Naturally,  is how much memory we need for 2  s, and so on.

So how do we  an array of ten  s like before? If we wish to declare and make room in one hit, we can simply say We only need to declare the pointer;  gives us some space to store the 10  s, and returns the pointer to the first element, which is assigned to that pointer.

Important note!  does not initialize the array; this means that the array may contain random or unexpected values! Like creating arrays without dynamic allocation, the programmer must initialize the array with sensible values before using it. Make sure you do so, too. (See later the function  for a simple method.)

It is not necessary to immediately call  after declaring a pointer for the allocated memory. Often a number of statements exist between the declaration and the call to, as follows:

A more practical example of dynamic memory allocation would be the following:"Given an array of 10 integers, remove all duplicate elements from the array, and create a new array without duplicate elements (a set)."A simple algorithm to remove duplicate elements: Because the length of our new array depends on the input, it must be dynamically allocated: The above array will currently contain unexpected values, so we must use  to set our dynamically allocated memory block to the new values:

Some security researchers recommend always using calloc(x,y) rather than malloc(x*y), for 2 reasons:
 * Many implementations of calloc carefully check the x and y arguments and return NULL if "x*y" could overflow. Using malloc(x*y), the multiplication "x*y" can overflow to 0 or some other too-small number, usually leading to buffer overflow.
 * calloc ensures that the buffer is completely empty of sensitive information, avoiding some kinds of security bugs (but, unfortunately, this would not have prevented the Heartbleed bug).

Error checking
When we want to use, we have to be mindful that the pool of memory available to the programmer is finite. Even if a modern PC will have at least an entire gigabyte of memory, it is still possible and conceivable to run out of it! In this case,  will return. In order to stop the program crashing from having no more memory to use, one should always check that malloc has not returned  before attempting to use the memory; we can do this by

Of course, suddenly quitting as in the above example is not always appropriate, and depends on the problem you are trying to solve and the architecture you are programming for. For example, if the program is a small, non critical application that's running on a desktop quitting may be appropriate. However if the program is some type of editor running on a desktop, you may want to give the operator the option of saving their tediously entered information instead of just exiting the program. A memory allocation failure in an embedded processor, such as might be in a washing machine, could cause an automatic reset of the machine. For this reason, many embedded systems designers avoid dynamic memory allocation altogether.

The function
The  function allocates space for an array of items and initializes the memory to zeros. The call  allocates   objects, each of whose size is sufficient to contain an instance of the structure. The space is initialized to all bits zero. The function returns either a pointer to the allocated memory or, if the allocation fails,.

The function
The  function changes the size of the object pointed to by   to the size specified by. The contents of the object shall be unchanged up to the lesser of the new and old sizes. If the new size is larger, the value of the newly allocated portion of the object is indeterminate. If  is a null pointer, the   function behaves like the   function for the specified size. Otherwise, if  does not match a pointer earlier returned by the ,  , or   function, or if the space has been deallocated by a call to the   or   function, the behavior is undefined. If the space cannot be allocated, the object pointed to by  is unchanged. If  is zero and   is not a null pointer, the object pointed to is freed. The  function returns either a null pointer or a pointer to the possibly moved allocated object.

The function
Memory that has been allocated using,  , or   must be released back to the system memory pool once it is no longer needed. This is done to avoid perpetually allocating more and more memory, which could result in an eventual memory allocation failure. Memory that is not released with  is however released when the current program terminates on most operating systems. Calls to  are as in the following example.

free with recursive data structures
It should be noted that  is neither intelligent nor recursive. The following code that depends on the recursive application of free to the internal variables of a struct does not work.

The statement " " will not free, causing a memory leak. The correct way is to define a function that frees every node in the data structure:

Because C does not have a garbage collector, C programmers are responsible for making sure there is a  exactly once for each time there is a. If a tree has been allocated one node at a time, then it needs to be freed one node at a time.

Don't free undefined pointers
Furthermore, using  when the pointer in question was never allocated in the first place often crashes or leads to mysterious bugs further along.

To avoid this problem, always initialize pointers when they are declared. Either use  at the point they are declared (as in most examples in this chapter), or set them to   when they are declared (as in the "delayed allocation" example in this chapter).

Write constructor/destructor functions
One way to get memory initialization and destruction right is to imitate object-oriented programming. In this paradigm, objects are constructed after raw memory is allocated for them, live their lives, and when it is time for them to be destructed, a special function called a destructor destroys the object's innards before the object itself is destroyed.

For example:

title: C Programming/Memory management


text:
Talk:Indian Malaysian

Moved
I've moved the Indian Malaysian page to Malaysian Indian.

This is because the article is about members of the Indian race who live in Malaysia. Therefore, the title "Malaysian Indian" is better suited for the article as it implies that we are Indians but we are of Malaysian citizenship.

Further discussions regerding this move can be discussed in the Talk Page of the new Malaysian Indian article.

Please do not revert this move before discussing in the talk page. This move was done only after discussion and agreement.

--Pavithran 07:35, 25 February 2007 (UTC) ---

title: Talk:Indian Malaysian


text:
File:Twiki screen 4 0 d.png

Summary
TWiki screenshot: Edit tables with EditTablePlugin. Author: Peter Thoeny (uploaded by author for TWiki page)

title: File:Twiki screen 4 0 d.png


text:
Talk:Dava Sobel

Pronunciation is missing
Hello. help me.

How can i pronounce 'dava sobel'?

 * Dae-vuh So-bell —The preceding unsigned comment was added by <IP_ADDRESS> (talk) 01:03, 12 January 2007 (UTC).

Galileo's Daughter
The article mentions the adaptation of Longitude, but there was also an adaption of Galileo's Daughter as a 2-hour Nova special. —Preceding unsigned comment added by <IP_ADDRESS> (talk) 04:35, 9 May 2009 (UTC)

List of books is incomplete
Ms. Sobel has written Langengrad in German (1999), Arthritis - The complete guide to relief using methods that really work (2005), and others. This article needs expansion. David Spector (talk) 22:01, 2 December 2016 (UTC)

External links modified
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I have just modified 1 one external link on Dava Sobel. Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:
 * Added archive https://web.archive.org/web/20111206022609/http://www.davasobel.com:80/about_bio.php to http://www.davasobel.com/about_bio.php

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title: Talk:Dava Sobel