Laws of Physics

Theories and Laws of Physics

Theories and laws are both used in science, but are also quite different from each other. This is also true with theories and laws. Theories do not become laws and laws were not once theories.

A theory is a set of ideas that try to explain things that we observe in the natural world. Theories are based on ideas that have been tested and proven to be true over time. This information comes from different scientists with different points of view, so they can be very biased until proven true through scientific experiments. This is important because different points of view help us discover lots of important information. Sometimes we learn new things because we have new instruments or make new discoveries. When this happens we might change our theory. Theories are always changing. However, theories do not ever become laws.

Like theories laws are based on what we observe and have learned from scientific experiments. Laws tell us about what happens in nature. They do not explain why things happen just that they do. One example of this is the law of gravity. This tells us that there is a force of attraction between two objects. It does not try to explain why there is a force. It just tells us that there is one. Laws were not once theories and theories do not become laws.

The basic laws of physics fall into two categories: classical physics that deals with the observable world (classical mechanics), and atomic physics that deals with the interactions between elementary and sub atomic particles (quantum mechanics). We will discuss each of the most common laws as we move through the different properties of physics.

Classical physics refers to theories of physics that predate modern, more complete, or more widely applicable theories.

Quantum mechanics is the science of the very small: the body of scientific principles that explains the behaviour of matter and its interactions with energy on the scale of atoms and subatomic particles.

Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. It is primarily concerned with the arrangement of electrons around the nucleus and the processes by which these arrangements change.

Murphy's Law (1942)

If anything can go wrong, it will.

What is Gravity?

Newton first started thinking about the Law of Gravity when an apple fell on his head. Gravity is a force pulling together all matter (which is anything you can physically touch). The more matter, the more gravity, so things that have a lot of matter such as planets and moons and stars pull more strongly.

Mass is how we measure the amount of matter in something. The more massive something is, the more of a gravitational pull it exerts. As we walk on the surface of the Earth, it pulls on us, and we pull back. But since the Earth is so much more massive than we are, the pull from us is not strong enough to move the Earth, while the pull from the Earth can make us fall flat on our faces.

In addition to depending on the amount of mass, gravity also depends on how far you are from something. This is why we are stuck to the surface of the Earth instead of being pulled off into the Sun, which has many more times the gravity of the Earth.

Newton's Law of universal gravitation

Two bodies attract each other with equal and opposite forces; the magnitude of this force is proportional to the product of the two masses and is also proportional to the inverse square of the distance between the centers of mass of the two bodies; F = (G m M/r2) e, where m and M are the masses of the two bodies, r is the distance between. the two, and e is a unit vector directed from the test mass to the second.

Gravity is one of the four fundamental forces, along with the electromagnetic, strong and weak forces. It is what causes objects to have weight. When you weigh yourself, the scale tells you how much gravity is acting on your body. The formula for determining weight is: weight equals mass times gravity. On Earth, gravity is a constant 9.8 meters per second squared, or 9.8 m/s2.

Historically, philosophers such as Aristotle thought that heavier objects accelerate toward the ground faster. But later experiments showed that wasn't the case. The reason that a feather will fall more slowly than a bowling ball is because of the drag from air resistance, which acts in the opposite direction as the acceleration due to gravity.

Newton's Law of Universal Gravitation says that the force of gravity is directly proportional to the product of their masses and inversely proportional to the square of the distance between them

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Sir Isaac Newton developed his Theory of Universal Gravitation in the 1680s. He found that gravity acts on all matter and is a function of both mass and distance. Every object attracts every other object with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them. The equation is often expressed as:

Fg = G (m1 ∙ m2) / r2

Fg is the gravitational force
m1 and m2 are the masses of the two objects
r is the distance between the two objects
G is the universal gravitational constant

Newton's equations work extremely well to predict how objects such as planets in the solar system behave.

Theory of relativity

Newton published his work on gravitation in 1687, which reigned as the best explanation until Einstein came up with his theory of general relativity in 1915. In Einstein's theory, gravity isn't a force, but rather, the consequence of the fact that matter warps space-time. One prediction of general relativity is that light will bend around massive objects.

Fun facts

The gravity on the moon is about 16 percent of that on Earth, Mars has about 38 percent of Earth's pull, while the biggest planet in the solar system, Jupiter, has 2.5 times the gravity of Earth.
Though nobody "discovered" gravity, legend has it that famous astronomer Galileo Galilei did some of the earliest experiments with gravity, dropping balls off the Tower of Pisa to see how fastthey fell.
Isaac Newton was just 23 years old and back from university when he noticed an apple falling in his garden and began unraveling the mysteries of gravity. (It's probably a myth that the apple bonked him on the head though.)
An early measure of Einstein's theory of relativity was the bending of starlight near the sun during a solar eclipse on May 29, 1919.
Black holes are massive collapsed stars with such strong gravity that even light cannot escape from it.
Einstein's theory of general relativity is incompatible with quantum mechanics, the bizarre laws that govern the behavior of the tiny particles — such as photons and electrons — that make up the universe.

Newtons Laws of Motion and Force

Newton's laws of motion are three physical laws that, together, laid the foundation for classical mechanics. They describe the relationship between a body and the forces acting upon it, and its motion in response to those forces. They have been expressed in several different ways, over nearly three centuries, and can be summarised as follows.

First law: When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.

Second law:The vector sum of the external forces F on an object is equal to the mass m of that object multiplied by the acceleration vectora of the object: F = ma.

Third law:When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

The three laws of motion were first compiled by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), first published in 1687.[4] Newton used them to explain and investigate the motion of many physical objects and systems.[5] For example, in the third volume of the text, Newton showed that these laws of motion, combined with his law of universal gravitation, explained Kepler's laws of planetary motion.

Newton's Laws of motion

Newton's first law of motion

A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

Newton's second law of motion

For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

Newton's third law of motion

In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction force.

Newton's laws are applied to objects which are idealised as single point masses,[6] in the sense that the size and shape of the object's body are neglected to focus on its motion more easily. This can be done when the object is small compared to the distances involved in its analysis, or thedeformation and rotation of the body are of no importance. In this way, even a planet can be idealised as a particle for analysis of its orbital motion around a star.

In their original form, Newton's laws of motion are not adequate to characterise the motion of rigid bodies and deformable bodies. Leonhard Euler in 1750 introduced a generalisation of Newton's laws of motion for rigid bodies called Euler's laws of motion, later applied as well for deformable bodies assumed as a continuum. If a body is represented as an assemblage of discrete particles, each governed by Newton's laws of motion, then Euler's laws can be derived from Newton's laws. Euler's laws can, however, be taken as axioms describing the laws of motion for extended bodies, independently of any particle structure.

Newton's laws hold only with respect to a certain set of frames of reference called Newtonian or inertial reference frames. Some authors interpret the first law as defining what an inertial reference frame is; from this point of view, the second law only holds when the observation is made from an inertial reference frame, and therefore the first law cannot be proved as a special case of the second. Other authors do treat the first law as a corollary of the second. The explicit concept of an inertial frame of reference was not developed until long after Newton's death.

In the given interpretation mass, acceleration, momentum, and (most importantly) force are assumed to be externally defined quantities. This is the most common, but not the only interpretation of the way one can consider the laws to be a definition of these quantities.

Newtonian mechanics has been superseded by special relativity, but it is still useful as an approximation when the speeds involved are much slower than the speed of light.

E=mc2

E = mc2, equation in German-born physicist Albert Einstein’s theory of special relativity that showed that the increased relativistic mass (m) of a body comes from the energy of motion of the body—that is, its kinetic energy (E)—divided by the speed of light squared (c2). This equation expresses the fact that mass and energy are the same physical entity and can be changed into each other.

In physics, mass–energy equivalence is the concept that the mass of an object or system is a measure of its energy content. For instance, adding 25 kilowatt-hours (90 megajoules) of any form of energy to any object increases its mass by 1 microgram (and, accordingly, its inertia and weight) even though no matter has been added.

A physical system has a property called energy and a corresponding property called mass; the two properties are equivalent in that they are always both present in the same (i.e. constant) proportion to one another. Mass–energy equivalence arose originally from special relativity as a paradox described by Henri Poincaré. The equivalence of energy E and mass m is reliant on the speed of light c and is described by the famous equation:

Thus, this mass–energy relation states that the universal proportionality factor between equivalent amounts of energy and mass is equal to the speed of light squared. This also serves to convert units of mass to units of energy, no matter what system of measurement units used.

If a body is stationary, it still has some internal or intrinsic energy, called its rest energy. Rest mass and rest energy are equivalent and remain proportional to one another. When the body is in motion (relative to an observer), its total energy is greater than its rest energy. The rest mass (or rest energy) remains an important quantity in this case because it remains the same regardless of this motion, even for the extreme speeds or gravity considered in special and general relativity; thus it is also called the invariant mass.

On the one hand, the equation E = mc2 can be applied to rest mass (m or m0) and rest energy (E0) to show their proportionality asE0 = m0c2.

On the other hand, it can also be applied to the total energy (Etot or simply E) and total mass of a moving body. The total mass is also called the relativistic mass mrel. The total energy and total mass are related by E = mrelc2.[3]

Thus, the mass–energy relation E = mc2 can be used to relate the rest energy to the rest mass, or to relate the total energy to the total mass. To instead relate the total energy or mass to the rest energy or mass, a generalization of the mass–energy relation is required: the energy–momentum relation.

E = mc2 has frequently been invoked as an explanation for the origin of energy in nuclear processes specifically, but such processes can be understood as converting nuclear potential energy in a manner precisely analogous to the way that chemical processes convert electrical potential energy. The more common association of mass–energy equivalence with nuclear processes derives from the fact that the large amounts of energy released in such reactions may exhibit enough mass that the mass loss (which is called themass defect) may be measured, when the released energy (and its mass) have been removed from the system; while the energy released in chemical processes is smaller by roughly six orders of magnitude, and so the resulting mass defect is much more difficult to measure. For example, the loss of mass to an atom and a neutron, as a result of the capture of the neutron and the production of a gamma ray, has been used to test mass–energy equivalence to high precision, as the energy of the gamma ray may be compared with the mass defect after capture. In 2005, these were found to agree to 0.0004%, the most precise test of the equivalence of mass and energy to date. This test was performed in the World Year of Physics 2005, a centennial celebration of Albert Einstein's achievements in 1905.[4]

Einstein was not the first to propose a mass–energy relationship (see the History section). However, Einstein was the first scientist to propose the E = mc2 formula and the first to interpret mass–energy equivalence as a fundamental principle that follows from therelativistic symmetries of space and time.

Top 10 Influential Figures in Science and Technology

Einsteins Theory of Relativity

In his special theory of relativity, Einstein showed that time and length are not as absolute as everyday experience would suggest: Moving clocks run slower, and moving objects are shorter. Those are just two of the unusual properties of Einstein's world! Another consequence of special relativity is the most famous formula of all: E=mc², stating that two physical quantities which physicists had defined separately, namely energy and mass, are in fact equivalent.

In Einstein's general theory of relativity, space and time become even more flexible. "Your mileage may vary," and so may the time intervals you measure, depending on where and when you are. This flexibility has an analogue in the geometry of surfaces like that of a sphere - there is a curvature of space and time. Distorted space and time influence the way that material objects or light move. In fact, there is a direct connection to the cosmic interaction that holds the universe together, makes the earth orbit the sun and keeps our feet on the ground: gravity.

Einstein's theory of space, time and gravity predicts a number of new phenomena. Distortions of the geometry of space should propagate into the depths of space as so-called gravitational waves. If enough mass is concentrated in a given location, the perfect geometrical prison should form - a region called a black hole. No object that enters such a region can ever escape! In addition, there are the big bang models, which form the foundation of modern cosmology - the study of the universe as a whole, its structure and evolution.

General relativity is the foundation of modern astrophysics and cosmology. But there is another physical theory at least as fundamental: quantum theory. Our section Relativity and the quantumtells you what happens when you combine quantum theory and Einstein's special relativity: the result is modern particle physics, the study of the most elementary constituents of matter. The same section takes you right to the frontiers of today's physics - more concretely, to one of its most persistent unsolved problems: There's still no complete theory of quantum gravity, i.e. no theory that unites Einstein's general relativity with the laws of the quantum world.

Electricity, magnetism & light

Coulomb´s law

Coulomb's Law is one of the basic ideas of electricity in physics. The law looks at the forces created between two charged objects. As distance increases, the forces and electric fields decrease. This simple idea was converted into a relatively simple formula. The force between the objects can be positive or negative depending on whether the objects are attractedto each other or repelled.

Charles Augustin de Coulomb was a French scientist working in the late 1700's. A little earlier, a British scientist named Henry Cavendish came up with similar ideas. Coulomb received most of the credit for the work on electric forces because Cavendish did not publish all of his work. The world never knew about Cavendish's work until decades after he died.

Think about a few concepts before you continue reading. Some charges are attracted to each other. Positive and negative charges like to move towards each other. Similar charges such as two positive or two negative push away from each other. You also need to understand that forces between objects become stronger as they move together and weaker as they move apart. You could yell at someone from far away, and they would barely hear you. If you yelled the same amount when you were together, it would be more powerful and loud.

Coulomb's Law

But you're here to learn about the law. When you have two charged particles, an electric force is created. If you have larger charges, the forces will be larger. If you use those two ideas, and add the fact that charges can attract and repel each other you will understand Coulomb's Law. It's a formula that measures the electrical forces between two objects.

Force between two charges = The constant of the equation (k) * The values of the amount of charge in each particle (q1 and q 2) / the distance between two charges (r or radius of seperation)

F = kq1q2/r2

k is Coulomb's constant (8.9875517873681764 x 10^9 N * m^2 * C^-2)

q1 and q2 are the signed magnitudes of the charges

r is the distance between the charges

"F" is the resulting force between the two charges. The distance between the two charges is "r." The "r" actually stands for "radius of separation" but you just need to know it is a distance. The "q1" and "q2" are values for the amount of charge in each of the particles. Scientists use Coulombs as units to measure charge. The constant of the equation is "k." As you learn more physics, you will see that this formula is very similar to a formula from Newton's work with gravity.

Coulomb's law states that:

The magnitude of the electrostatic force of interaction between two point charges is directly proportional to the scalar multiplication of the magnitudes of charges and inversely proportional to the square of the distance between them.[12]

The force is along the straight line joining them. If the two charges have the same sign, the electrostatic force between them is repulsive; if they have different signs, the force between them is attractive.