In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass i.e. to accelerate. Force can be described intuitively as a push or a pull. A force has both direction, making it a vector quantity, it is measured in the SI unit of newtons and represented by the symbol F. The original form of Newton's second law states that the net force acting upon an object is equal to the rate at which its momentum changes with time. If the mass of the object is constant, this law implies that the acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, is inversely proportional to the mass of the object. Concepts related to force include: thrust. In an extended body, each part applies forces on the adjacent parts; such internal mechanical stresses cause no acceleration of that body as the forces balance one another. Pressure, the distribution of many small forces applied over an area of a body, is a simple type of stress that if unbalanced can cause the body to accelerate.

Stress causes deformation of solid materials, or flow in fluids. Philosophers in antiquity used the concept of force in the study of stationary and moving objects and simple machines, but thinkers such as Aristotle and Archimedes retained fundamental errors in understanding force. In part this was due to an incomplete understanding of the sometimes non-obvious force of friction, a inadequate view of the nature of natural motion. A fundamental error was the belief that a force is required to maintain motion at a constant velocity. Most of the previous misunderstandings about motion and force were corrected by Galileo Galilei and Sir Isaac Newton. With his mathematical insight, Sir Isaac Newton formulated laws of motion that were not improved for nearly three hundred years. By the early 20th century, Einstein developed a theory of relativity that predicted the action of forces on objects with increasing momenta near the speed of light, provided insight into the forces produced by gravitation and inertia.

With modern insights into quantum mechanics and technology that can accelerate particles close to the speed of light, particle physics has devised a Standard Model to describe forces between particles smaller than atoms. The Standard Model predicts that exchanged particles called gauge bosons are the fundamental means by which forces are emitted and absorbed. Only four main interactions are known: in order of decreasing strength, they are: strong, electromagnetic and gravitational. High-energy particle physics observations made during the 1970s and 1980s confirmed that the weak and electromagnetic forces are expressions of a more fundamental electroweak interaction. Since antiquity the concept of force has been recognized as integral to the functioning of each of the simple machines; the mechanical advantage given by a simple machine allowed for less force to be used in exchange for that force acting over a greater distance for the same amount of work. Analysis of the characteristics of forces culminated in the work of Archimedes, famous for formulating a treatment of buoyant forces inherent in fluids.

Aristotle provided a philosophical discussion of the concept of a force as an integral part of Aristotelian cosmology. In Aristotle's view, the terrestrial sphere contained four elements that come to rest at different "natural places" therein. Aristotle believed that motionless objects on Earth, those composed of the elements earth and water, to be in their natural place on the ground and that they will stay that way if left alone, he distinguished between the innate tendency of objects to find their "natural place", which led to "natural motion", unnatural or forced motion, which required continued application of a force. This theory, based on the everyday experience of how objects move, such as the constant application of a force needed to keep a cart moving, had conceptual trouble accounting for the behavior of projectiles, such as the flight of arrows; the place where the archer moves the projectile was at the start of the flight, while the projectile sailed through the air, no discernible efficient cause acts on it.

Aristotle was aware of this problem and proposed that the air displaced through the projectile's path carries the projectile to its target. This explanation demands a continuum like air for change of place in general. Aristotelian physics began facing criticism in medieval science, first by John Philoponus in the 6th century; the shortcomings of Aristotelian physics would not be corrected until the 17th century work of Galileo Galilei, influenced by the late medieval idea that objects in forced motion carried an innate force of impetus. Galileo constructed an experiment in which stones and cannonballs were both rolled down an incline to disprove the Aristotelian theory of motion, he showed that the bodies were accelerated by gravity to an extent, independent of their mass and argued that objects retain their velocity unless acted on by a force, for example friction. Sir Isaac Newton described the motion of all objects using the concepts of inertia and force, in doing so he found they obey certain conservation laws.

In 1687, Newton published his thesis Philosophiæ Naturalis Principia Mathematica. In this work Newton set out three laws of motion that to this day are t


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Aroup Chatterjee

Aroup Chatterjee is a British Indian author and physician. He was born in Calcutta, moved to the United Kingdom in 1985, he is the author of the book Mother Teresa: The Untold Story, a work which challenges the widespread regard of Mother Teresa as a symbol of philanthropy and selflessness. Chatterjee's criticism inspired the documentary Hell's Angel, shown on Channel 4, a British television channel; the documentary was written by a well-known critic of Mother Teresa, Christopher Hitchens, who co-produced it with journalist and filmmaker Tariq Ali. Chatterjee and Hitchens were the two Devil's advocates, or hostile witnesses to Catholic Church procedures for the beatification of Mother Teresa in 2003. Chatterjee was born in 1958 and raised in the city of Kolkata, West Bengal, moving to the United Kingdom in 1985. In the 1970s and 1980s while studying at Calcutta Medical College he worked part-time for a left-wing political party campaigning against poverty and worked at a hospital where he treated patients from the oldest and poorest districts of the city as well as refugees from the civil war with what is now Bangladesh.

While living in the UK he became concerned by the common portrayal of the widespread destitution and disease in his native Calcutta which stemmed from press reporting of the work of Mother Teresa. At that point he describes his attitude to Mother Teresa as "If anything, I was positively inclined towards her" although he says he never saw any of her nuns in the slums; however it was this image at odds with his own experience as a doctor in Calcutta that caused him to look more at her work and reputation. From the 1990s onwards he began to uncover what he calls a "cult of suffering" which Mother Teresa and her followers in the Missionaries of Charity were running back in Calcutta supported by her friend Pope John Paul II. In February 1993 Chatterjee sent a proposal for a short documentary to Vanya Del Borgo, associate producer of Bandung Productions, owned by Tariq Ali; the proposal was passed to a Channel 4 commissioner who approved it, Del Borgo with Chatterjee's proposal began work, approaching journalist and author Christopher Hitchens to write and present it.

The documentary became the 1994 film Hell's Angel. Chatterjee found the documentary "too sensationalist" and Hitchens went on to write his book The Missionary Position. Chatterjee spent the next year travelling and interviewing people who had worked with Mother Teresa and the Missionaries of Charity and began to campaign against the conditions in Nirmal Hriday known as the Kalighat Home for the Dying in Calcutta. In particular he heard stories of lack of basic hygiene, the absence of any pain medication and the frequent reuse of hypodermic needles. Chatterjee began work on a book released by Meteor Books in 2002 under the original title Mother Teresa: The Final Verdict. Chatterjee says in addition to the hours of interviews, "I started in pre Internet days and I spent months in libraries in London. I travelled the world researching it. I followed slum dwellers, destitute children with a video camera. I interviewed hundreds of people. I stood with video camera outside Teresa's home for hours."Following the publication of his book Chatterjee continued to speak out against what he calls the "bogus and fantastic figure" of Mother Teresa, acting as Devil's advocate in the process of her sainthood.

He continues to work as a physician in London where he lives with his Irish wife, raised as a Roman Catholic, their three children. In December 2002 independent publisher Meteor Books, owned by Bhagbat Chakraborty, published Chatterjee's book under the title Mother Teresa: The Final Verdict. In 2016 the same book was reissued under a new title, Mother Teresa: The Untold Story by Fingerprint! publishers after being taken up by literary agent Kanishka Gupta. The book covers her life and her rise to fame following the documentary Something Beautiful for God by Malcolm Muggeridge, the Calcutta home for the dying and the practices of running it, the non-consensual death bed baptisms of Hindus and Muslims, the pathetic hygienic practices in the homes run by her, her limited connections with Kolkata and the masses and the vast amount of financial donations given to the charity but not spent at Nirmal Hriday, he covers her Nobel Peace Prize and the speech in which she claimed to have saved tens of thousands of destitute people.

He writes about the celebrities and the powerful people who had audiences with her, the controversies surrounding the money she accepted from dictators such as Haitian president Jean-Claude Duvalier, convicted fraudster Charles Keating and disgraced publisher Robert Maxwell. The book looks at the worldwide reach of the Missionaries of Charity and examines the available evidence for her financial accounts along with her personal crusade against abortion and contraception, he blames the West the United States of America, for creating her benevolent image as a savior in the backdrops of a ravaged sub continent. The final chapters address her death and beatification and Chatterjee's own involvement as an official devil's advocate or hostile witness and the transcripts of the proceedings. Chatterjee sums up his view of Mother Teresa's life's work as: The self-published book has been praised for the content but has been criticized for its editorial errors. Times Higher Education praised the book as necessary and well-documented, which could have been improved with editing.

The Irish Times praised the content and advocated for its wide dissemination in light of its seriousness but noted Chatterjee's personal agenda t