# All physics explained in 15 minutes (worth remembering)

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Five areas of physics worth remembering: Classical mechanics, energy and thermodynamics, electromagnetism, Relativity, and Quantum Mechanics. Classical mechanics - two main concepts worth knowing. The first is Newton’s second law: F= ma: Force equals mass times acceleration. If you apply a force to a fixed mass, it tells you how much acceleration you will get. And knowing acceleration which is the change in velocity, you can make predictions.

The second equation is the law of universal gravitation. it allows us to determine the motion of heavenly bodies. It says that the gravitational attraction between two bodies is the product of their masses divided by the distance between them squared, times a constant, called Newton’s gravitational constant.

Energy is not a vector like force or momentum, but it is just a number. Work is closely related to energy. It is force times distance traveled. Energy for most objects consists of kinetic energy plus potential energy. KE is the energy of motion, It is KE = ½ M V^2 – the more mass you have and/or the more velocity you have, the more energy you have.

Gravitational potential energy is expressed as PE = m g h – mass times the gravitational acceleration times the height. The total energy of an object is both Kinetic energy plus potential energy. Potential energy can take many forms. Gasoline or petrol has chemical potential energy. Important: Energy is always conserved. It is not created or destroyed. It only changes form.

Thermodynamics is the study of work, heat, and energy on a system. We showed energy is how much work you could do. But another form of energy is thermal energy. If a car is moving and you apply the brakes, the kinetic energy of the car gets converted to thermal energy, created by friction of the car’s brakes. Temperature is the average kinetic energy of atoms in a system. Thermal energy is the total kinetic energy of atoms in a system.

Entropy is a measure of disorder, or more accurately, the information required to describe the micro states of a system. The 2nd law of thermodynamics states that entropy of an isolated system can never decrease. Energy at lower entropy can do more work than energy at high entropy. The one way flow of Entropy seems to be the only reason we have a forward flow of time.

Electromagnetism is the study of the interaction between electrically charged particles. The essentials are in Maxwell’s equations. If you have a static object with a charge, it will affect only other charges. If you have a static magnet, it will affect only other magnets. It will not affect charges. But if you have a moving charge, it will affect a magnet. And if you have a moving magnet, it will affect a charge. The constants mu naught and epsilon naught are the permeability and permittivity of free space. These two constants determine the speed of light because they measure the resistance of space to changing electric and magnetic fields.

Special Relativity: Einstein presumed that the speed is the same in any frame of reference. This was one of the postulates.

The second postulate was principle of relativity - the laws of physics are the same for all observes who are moving at the same velocity relative to each other. Einstein showed that the only way these can be true is if time was not fixed, but was relative.

General relativity: Later Einstein showed using the same assumptions, there would be no way to tell if you were in an accelerating reference frame or standing stationary on earth. A flashlight beam will bend in gravity. But since light always takes the shortest path between any two points, this means that space-time itself is bending.

Quantum mechanics: Three principles are important. First by Max Planck, says that energy is not continuous, but is quantized. The amount of energy equals the frequency of the radiation times Planck's constant. Using this, Einstein later showed that a photon is both a wave and a particle.

The second is the Heisenberg's uncertainty principle: you cannot know both a particle’s exact position and it’s exact momentum at the same time. For a particle with mass, this means if you know exactly where a particle is, you don’t know how fast going. If you know exactly how fast it’s going, you don’t know where it is.

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Schrodinger's equation: prior to measurement, quantum systems are in superposed states. This means that their properties can only be expressed as a wave function. A wave function simplified, is a set of probabilities. So in a hydrogen atom, you can’t know where to find the electron in advance. All you can know is the probability of where you might find it, if you measured it. Prior to measurement, all quantum systems are waves of probabilities. This is not a limitation of our measuring devices. It is a limitation of reality.

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