The Origin of All Forces

What is normal force?

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This normal force will be as large as necessary to prevent the surfaces from penetrating each other. The mechanism by which the table exerts a normal force is somewhat similar to how a compressed spring exerts a force. When a weight is placed on top of the table, the surface deforms slightly and changes shape usually imperceptibly unless the weight is very large and there is a restoring force that tries to restore the table to its natural and uncompressed shape i.

This force the table exerts to restore itself is what we call the normal force. If you place enough force on the table, the large deformation could break the table. Of course, the box would also get compressed slightly and try to restore itself to its natural shape. Now, the actual deformations for objects in contact under typical non-extreme conditions are usually minuscule so any change in shape or volume can almost always be ignored for a given problem.

The word "normal" in normal force is not referring to ordinary or commonplace. The "normal" here refers to perpendicular. It makes sense that the force is perpendicular to the surface since the normal force is what prevents solid objects from passing through each other. Surfaces can also exert contact forces in the direction parallel to the surfaces, but we would typically call those forces frictional forces since they work to prevent the surfaces from sliding across each other instead of calling them normal forces. How do inanimate surfaces "know" to exert a normal force?

It makes sense for most people that a person would have to exert an upward force with their hands when carrying a heavy bag of dog food as seen in Figure 3 a below. But some people find it hard to believe that an inanimate object like a table can exert an upward normal force on a bag of dog food as seen in Figure 3 b seen below. Sometimes people believe that the table is not really exerting an upward force at all, but merely "getting in the way" of the dog food falling down. But that's not how Newton's laws work. If there was only a downward force of gravity on the dog food, the dog food would have to accelerate downward.

The table must do more than "get in the way". The table must exert an upward force to prevent the dog food from falling through the table. Strangely, if a heavier object is placed on a table, the table must exert more normal force to prevent the weight from passing through the table. How does the table know to exert just the right amount of force to prevent the object from passing through it? When solid objects deform they typically try to restore themselves and "spring back" to their natural shape.

The heavier the weight, the greater the deformation, the greater the restoring force trying to bring the surface back to its natural shape. This deformation would be noticeable if the load were placed on a card table, but even rigid objects deform when a force is applied to them. Unless the object is deformed beyond its limit, it will exert a restoring force much like a deformed spring or trampoline or diving board.

Faraday conjectured that ultimately, all forces unified into one. In , James Clerk Maxwell unified electricity and magnetism as effects of an electromagnetic field whose third consequence was light, travelling at constant speed in a vacuum. The electromagnetic field theory contradicted predictions of Newton's theory of motion, unless physical states of the luminiferous aether —presumed to fill all space whether within matter or in a vacuum and to manifest the electromagnetic field—aligned all phenomena and thereby held valid the Newtonian principle relativity or invariance.

The Standard Model of particle physics was developed throughout the latter half of the 20th century. In the Standard Model, the electromagnetic, strong, and weak interactions associate with elementary particles , whose behaviours are modelled in quantum mechanics QM. For predictive success with QM's probabilistic outcomes, particle physics conventionally models QM events across a field set to special relativity , altogether relativistic quantum field theory QFT.

Everyday matter is atoms, composed of three fermion types: Atoms interact, form molecules , and manifest further properties through electromagnetic interactions among their electrons absorbing and emitting photons, the electromagnetic field's force carrier, which if unimpeded traverse potentially infinite distance. The electromagnetic interaction was modelled with the weak interaction, whose force carriers are W and Z bosons , traversing the minuscule distance, in electroweak theory EWT.

Electroweak interaction would operate at such high temperatures as soon after the presumed Big Bang , but, as the early universe cooled, split into electromagnetic and weak interactions. The strong interaction, whose force carrier is the gluon , traversing minuscule distance among quarks, is modeled in quantum chromodynamics QCD. Predictions are usually made using calculational approximation methods, although such perturbation theory is inadequate to model some experimental observations for instance bound states and solitons.

Still, physicists widely accept the Standard Model as science's most experimentally confirmed theory. Some attempts at GUTs hypothesize "shadow" particles, such that every known matter particle associates with an undiscovered force particle , and vice versa, altogether supersymmetry SUSY. Other theorists seek to quantize the gravitational field by the modelling behaviour of its hypothetical force carrier, the graviton and achieve quantum gravity QG.

The most prevalent aim at a ToE is string theory , although to model matter particles , it added SUSY to force particles —and so, strictly speaking, became superstring theory. Multiple, seemingly disparate superstring theories were unified on a backbone, M-theory.

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Theories beyond the Standard Model remain highly speculative, lacking great experimental support. They attract or repel each other by exchanging bosons. The exchange of bosons always carries energy and momentum between the fermions, thereby changing their speed and direction. The exchange may also transport a charge between the fermions, changing the charges of the fermions in the process e. Because an interaction results in fermions attracting and repelling each other, an older term for "interaction" is force.

According to the present understanding, there are four fundamental interactions or forces: Their magnitude and behaviour vary greatly, as described in the table below. Modern physics attempts to explain every observed physical phenomenon by these fundamental interactions.

Moreover, reducing the number of different interaction types is seen as desirable. Two cases in point are the unification of:. Both magnitude "relative strength" and "range", as given in the table, are meaningful only within a rather complex theoretical framework. It should also be noted that the table below lists properties of a conceptual scheme that is still the subject of ongoing research. The modern perturbative quantum mechanical view of the fundamental forces other than gravity is that particles of matter fermions do not directly interact with each other, but rather carry a charge, and exchange virtual particles gauge bosons , which are the interaction carriers or force mediators.

For example, photons mediate the interaction of electric charges , and gluons mediate the interaction of color charges. Gravitation is by far the weakest of the four interactions at the atomic scale, where electromagnetic interactions dominate. But the idea that the weakness of gravity can easily be demonstrated by suspending a pin using a simple magnet such as a refrigerator magnet is fundamentally flawed. The only reason the magnet is able to hold the pin against the gravitational pull of the entire Earth is due to its relative proximity.

There is clearly a short distance of separation between magnet and pin where a breaking point is reached, and due to the large mass of Earth this distance is disappointingly small. Thus gravitation is very important for macroscopic objects and over macroscopic distances for the following reasons.

Even though electromagnetism is far stronger than gravitation, electrostatic attraction is not relevant for large celestial bodies, such as planets, stars, and galaxies, simply because such bodies contain equal numbers of protons and electrons and so have a net electric charge of zero. Nothing "cancels" gravity, since it is only attractive, unlike electric forces which can be attractive or repulsive.

On the other hand, all objects having mass are subject to the gravitational force, which only attracts. Therefore, only gravitation matters on the large-scale structure of the universe. The long range of gravitation makes it responsible for such large-scale phenomena as the structure of galaxies and black holes and it retards the expansion of the universe.

Gravitation was the first interaction to be described mathematically. In ancient times, Aristotle hypothesized that objects of different masses fall at different rates. During the Scientific Revolution , Galileo Galilei experimentally determined that this hypothesis was wrong under certain circumstances — neglecting the friction due to air resistance, and buoyancy forces if an atmosphere is present e.

Isaac Newton's law of Universal Gravitation was a good approximation of the behaviour of gravitation. Our present-day understanding of gravitation stems from Einstein's General Theory of Relativity of , a more accurate especially for cosmological masses and distances description of gravitation in terms of the geometry of spacetime. Merging general relativity and quantum mechanics or quantum field theory into a more general theory of quantum gravity is an area of active research. It is hypothesized that gravitation is mediated by a massless spin-2 particle called the graviton.

Although general relativity has been experimentally confirmed at least for weak fields [ which? Those taken seriously by [ citation needed ] the physics community all reduce to general relativity in some limit, and the focus of observational work is to establish limitations on what deviations from general relativity are possible.

What is normal force?

Proposed extra dimensions could explain why the gravity force is so weak. 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: 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 ultimately culminated in the work of Archimedes who was especially 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 mostly 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" e. The place where the archer moves the projectile was at the start of the flight, and 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 fully corrected until the 17th century work of Galileo Galilei , who was 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 that was 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, and in doing so he found they obey certain conservation laws.

Newton's First Law of Motion states that objects continue to move in a state of constant velocity unless acted upon by an external net force resultant force. Newton proposed that every object with mass has an innate inertia that functions as the fundamental equilibrium "natural state" in place of the Aristotelian idea of the "natural state of rest". That is, Newton's empirical First Law contradicts the intuitive Aristotelian belief that a net force is required to keep an object moving with constant velocity.

By making rest physically indistinguishable from non-zero constant velocity , Newton's First Law directly connects inertia with the concept of relative velocities. Specifically, in systems where objects are moving with different velocities, it is impossible to determine which object is "in motion" and which object is "at rest". The laws of physics are the same in every inertial frame of reference , that is, in all frames related by a Galilean transformation.

Discussion

For instance, while traveling in a moving vehicle at a constant velocity , the laws of physics do not change as a result of its motion. If a person riding within the vehicle throws a ball straight up, that person will observe it rise vertically and fall vertically and not have to apply a force in the direction the vehicle is moving. Another person, observing the moving vehicle pass by, would observe the ball follow a curving parabolic path in the same direction as the motion of the vehicle. It is the inertia of the ball associated with its constant velocity in the direction of the vehicle's motion that ensures the ball continues to move forward even as it is thrown up and falls back down.

From the perspective of the person in the car, the vehicle and everything inside of it is at rest: It is the outside world that is moving with a constant speed in the opposite direction of the vehicle. Since there is no experiment that can distinguish whether it is the vehicle that is at rest or the outside world that is at rest, the two situations are considered to be physically indistinguishable.

Inertia therefore applies equally well to constant velocity motion as it does to rest. A modern statement of Newton's Second Law is a vector equation: If a body is in equilibrium, there is zero net force by definition balanced forces may be present nevertheless. In contrast, the second law states that if there is an unbalanced force acting on an object it will result in the object's momentum changing over time.

By the definition of momentum ,. If Newton's second law is applied to a system of constant mass , [Note 2] m may be moved outside the derivative operator. The equation then becomes. By substituting the definition of acceleration , the algebraic version of Newton's Second Law is derived:. Newton never explicitly stated the formula in the reduced form above. Newton's Second Law asserts the direct proportionality of acceleration to force and the inverse proportionality of acceleration to mass. Accelerations can be defined through kinematic measurements.

However, while kinematics are well-described through reference frame analysis in advanced physics, there are still deep questions that remain as to what is the proper definition of mass. General relativity offers an equivalence between space-time and mass, but lacking a coherent theory of quantum gravity , it is unclear as to how or whether this connection is relevant on microscales. With some justification, Newton's second law can be taken as a quantitative definition of mass by writing the law as an equality; the relative units of force and mass then are fixed.

The use of Newton's Second Law as a definition of force has been disparaged in some of the more rigorous textbooks, [4]: Notable physicists, philosophers and mathematicians who have sought a more explicit definition of the concept of force include Ernst Mach and Walter Noll.

Newton's Second Law can be used to measure the strength of forces. For instance, knowledge of the masses of planets along with the accelerations of their orbits allows scientists to calculate the gravitational forces on planets. Whenever one body exerts a force on another, the latter simultaneously exerts an equal and opposite force on the first. Newton's Third Law is a result of applying symmetry to situations where forces can be attributed to the presence of different objects. The third law means that all forces are interactions between different bodies, [15] [Note 3] and thus that there is no such thing as a unidirectional force or a force that acts on only one body.

In a system composed of object 1 and object 2, the net force on the system due to their mutual interactions is zero:. More generally, in a closed system of particles, all internal forces are balanced. The particles may accelerate with respect to each other but the center of mass of the system will not accelerate. If an external force acts on the system, it will make the center of mass accelerate in proportion to the magnitude of the external force divided by the mass of the system.

Combining Newton's Second and Third Laws, it is possible to show that the linear momentum of a system is conserved. Using similar arguments, this can be generalized to a system with an arbitrary number of particles. In general, as long as all forces are due to the interaction of objects with mass, it is possible to define a system such that net momentum is never lost nor gained.

In the special theory of relativity , mass and energy are equivalent as can be seen by calculating the work required to accelerate an object. When an object's velocity increases, so does its energy and hence its mass equivalent inertia. It thus requires more force to accelerate it the same amount than it did at a lower velocity. Relativistic force does not produce a constant acceleration, but an ever-decreasing acceleration as the object approaches the speed of light. Since forces are perceived as pushes or pulls, this can provide an intuitive understanding for describing forces.

Fundamental interaction

Through experimentation, it is determined that laboratory measurements of forces are fully consistent with the conceptual definition of force offered by Newtonian mechanics. Forces act in a particular direction and have sizes dependent upon how strong the push or pull is. Because of these characteristics, forces are classified as " vector quantities ". This means that forces follow a different set of mathematical rules than physical quantities that do not have direction denoted scalar quantities.

For example, when determining what happens when two forces act on the same object, it is necessary to know both the magnitude and the direction of both forces to calculate the result. If both of these pieces of information are not known for each force, the situation is ambiguous. For example, if you know that two people are pulling on the same rope with known magnitudes of force but you do not know which direction either person is pulling, it is impossible to determine what the acceleration of the rope will be.

The two people could be pulling against each other as in tug of war or the two people could be pulling in the same direction. In this simple one-dimensional example, without knowing the direction of the forces it is impossible to decide whether the net force is the result of adding the two force magnitudes or subtracting one from the other. Associating forces with vectors avoids such problems. Historically, forces were first quantitatively investigated in conditions of static equilibrium where several forces canceled each other out.

Such experiments demonstrate the crucial properties that forces are additive vector quantities: However, if the forces are acting on an extended body, their respective lines of application must also be specified in order to account for their effects on the motion of the body. Free-body diagrams can be used as a convenient way to keep track of forces acting on a system. Ideally, these diagrams are drawn with the angles and relative magnitudes of the force vectors preserved so that graphical vector addition can be done to determine the net force. As well as being added, forces can also be resolved into independent components at right angles to each other.

A horizontal force pointing northeast can therefore be split into two forces, one pointing north, and one pointing east. Summing these component forces using vector addition yields the original force. Resolving force vectors into components of a set of basis vectors is often a more mathematically clean way to describe forces than using magnitudes and directions.

Orthogonal components are independent of each other because forces acting at ninety degrees to each other have no effect on the magnitude or direction of the other. Choosing a set of orthogonal basis vectors is often done by considering what set of basis vectors will make the mathematics most convenient.

Choosing a basis vector that is in the same direction as one of the forces is desirable, since that force would then have only one non-zero component. Orthogonal force vectors can be three-dimensional with the third component being at right-angles to the other two. Equilibrium occurs when the resultant force acting on a point particle is zero that is, the vector sum of all forces is zero. When dealing with an extended body, it is also necessary that the net torque be zero. There are two kinds of equilibrium: Static equilibrium was understood well before the invention of classical mechanics.

Normal force and contact force

Objects that are at rest have zero net force acting on them. The simplest case of static equilibrium occurs when two forces are equal in magnitude but opposite in direction. For example, an object on a level surface is pulled attracted downward toward the center of the Earth by the force of gravity. At the same time, a force is applied by the surface that resists the downward force with equal upward force called a normal force. The situation produces zero net force and hence no acceleration.

Pushing against an object that rests on a frictional surface can result in a situation where the object does not move because the applied force is opposed by static friction , generated between the object and the table surface. For a situation with no movement, the static friction force exactly balances the applied force resulting in no acceleration.

The static friction increases or decreases in response to the applied force up to an upper limit determined by the characteristics of the contact between the surface and the object. A static equilibrium between two forces is the most usual way of measuring forces, using simple devices such as weighing scales and spring balances.

For example, an object suspended on a vertical spring scale experiences the force of gravity acting on the object balanced by a force applied by the "spring reaction force", which equals the object's weight. Using such tools, some quantitative force laws were discovered: These were all formulated and experimentally verified before Isaac Newton expounded his Three Laws of Motion.

Dynamic equilibrium was first described by Galileo who noticed that certain assumptions of Aristotelian physics were contradicted by observations and logic. Galileo realized that simple velocity addition demands that the concept of an "absolute rest frame " did not exist. Galileo concluded that motion in a constant velocity was completely equivalent to rest.

This was contrary to Aristotle's notion of a "natural state" of rest that objects with mass naturally approached. Simple experiments showed that Galileo's understanding of the equivalence of constant velocity and rest were correct. For example, if a mariner dropped a cannonball from the crow's nest of a ship moving at a constant velocity, Aristotelian physics would have the cannonball fall straight down while the ship moved beneath it. Thus, in an Aristotelian universe, the falling cannonball would land behind the foot of the mast of a moving ship. However, when this experiment is actually conducted, the cannonball always falls at the foot of the mast, as if the cannonball knows to travel with the ship despite being separated from it.

Since there is no forward horizontal force being applied on the cannonball as it falls, the only conclusion left is that the cannonball continues to move with the same velocity as the boat as it falls. Thus, no force is required to keep the cannonball moving at the constant forward velocity.

Moreover, any object traveling at a constant velocity must be subject to zero net force resultant force. This is the definition of dynamic equilibrium: A simple case of dynamic equilibrium occurs in constant velocity motion across a surface with kinetic friction. In such a situation, a force is applied in the direction of motion while the kinetic friction force exactly opposes the applied force. This results in zero net force, but since the object started with a non-zero velocity, it continues to move with a non-zero velocity.

Aristotle misinterpreted this motion as being caused by the applied force. However, when kinetic friction is taken into consideration it is clear that there is no net force causing constant velocity motion.

Forces – The Physics Hypertextbook

This has the consequence that the results of a measurement are now sometimes "quantized", i. This is, of course, difficult to imagine in the context of "forces". However, the potentials V x , y , z or fields , from which the forces generally can be derived, are treated similarly to classical position variables, i. This becomes different only in the framework of quantum field theory , where these fields are also quantized. However, already in quantum mechanics there is one "caveat", namely the particles acting onto each other do not only possess the spatial variable, but also a discrete intrinsic angular momentum-like variable called the " spin ", and there is the Pauli exclusion principle relating the space and the spin variables.

Depending on the value of the spin, identical particles split into two different classes, fermions and bosons.

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