Physics

## Length

Length is the most basic property of a one-dimensional object. It is measured using metres (which is the SI unit), inches, feet, or miles.

## Area

Area is the logical extension of the concept of length; it's two-dimensional rather than one-dimensional. It is measured in the same units as length, except that it uses those units, squared. For example, the area of something might be measured in square feet.

## Time

Time is most often measured in seconds or minutes.

## Force

A force is something that causes an object to move. The SI unit of force is the newton.

Newton's First Law of Motion states that an object at rest will remain at rest until a force is impressed upon that object, at which point it will begin to move. Similarly, an object in motion will continue moving in the same direction, at the same speed, unless a force acts upon it.

To be more specific, however, this law is referring to net force, not just any old force. Net force is the sum of all forces acting upon an object at any one time. For example, one force that acts upon almost any moving object in our world is air resistance, known as drag. Anything that moves through the atmosphere is pushing against the air, and the air tends to slow it down somewhat. The faster something is moving, the harder air pushes back against it. However, things can still move through the air, they just meet some resistance from the air along the way. If you're driving a car and you push harder on the gas pedal, the car will speed up, even though air is pushing against it. This is because the engine is stronger than the air resistance. The net force acting on the car includes both the force of the engine and the air drag.

In our world, even unmoving objects are always being acted upon by at least one force: Gravity. However, if an object is resting on a solid surface (such as a table), the surface is pushing up on the object just as much as gravity is pulling down on the object. Because one force is pulling down on the object, and the other is pushing in the opposite direction with the same amount of force, the net force upon the object is zero, and therefore it remains still. It will continue to remain at rest until a non-zero net force is impressed upon it.

## Speed

Speed is not usually measured in a dedicated unit. Rather, it is measured as a function of distance with regard to time. For example, "miles per hour" is a common measure of speed.

Speed = distance / time

## Velocity

Velocity and speed are often confused. In fact, sometimes the terms are used interchangeably, but technically, there is a difference: Speed is merely a quantity, or what is mathematically called a scalar Velocity, on the other hand, is a vector: It includes both a magnitude and a direction. Thus, a velocity is both a speed and a direction. So, for example, "30 miles per hour eastward" is a velocity.

Velocity = speed with a direction vector

A constant velocity is a velocity that's not changing. If something is moving at a constant speed, in a constant direction, it has a constant velocity. If something is changing its speed, it does not have a constant velocity. Similarly, if something is not moving in a straight line but it is maintaining speed, then it has a constant speed, but NOT a constant velocity. Objects which maintain a constant velocity are sometimes said to be in mechanical equilibrium.

## Acceleration

An acceleration is a change in velocity. If something changes its velocity--either its speed or its direction--then it has experienced an acceleration.

The term "acceleration" is most often associated with speed, but it can also refer to direction. Even if something is moving at a constant speed, if it is moving in a circle (for example), it is accelerating.

Newton's Second Law of Motion states that the acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to the object's mass. What this basically means is that the harder an object is pushed or pulled, the more it will accelerate, and the more mass the object has, the less it will accelerate. Acceleration can be expressed as a simple formula:

Acceleration = net force / mass

The units used to measure acceleration can be a bit difficult to understand at first. Acceleration is measured in terms of velocity per unit of time. However, recall that speed is already expressed in terms of distance per unit of time. Therefore, acceleration is measured in distance per unit of time per unit of time. For example, an object may be accelerating at a rate of one meter per second per second; this means that every second, the object is speeding up by one meter per second (1 m/s). After one second, the object will be moving at 1 m/s. After two seconds, the object will be moving at 2 m/s. After three seconds, the object will be moving at 3 m/s. The object's acceleration is constant, but its speed is not. The object's acceleration is 1 m/s/s. The unit "meters per second per second" is the same as "meters per second squared", which is the term preferred by physicists.

## Gravity

Gravity is an acceleration, not a constant speed. If something falls through the air, it doesn't fall at a constant speed. It keeps falling faster and faster as time goes by.

The acceleration of gravity is constant. It is an acceleration of 9.8 metres per second per second. This becomes less as one moves away from the Earth, but near the surface of the Earth, this acceleration is constant.

Gravity = 9.8 m/s/s

## Volume

Volume is a three-dimensional extension of the concepts of length and area. It is measured in the units of length, cubed. For example, the volume of a sphere might be measured in cubic feet.

## Mass

Mass is a measure of how much matter exists within an object. It is often confused with weight.

The SI unit of mass is the gram.

## Weight

An object's weight is how hard it is pulled down by gravity. This is a product of two factors: Mass and gravity. All objects with the same mass will have the same weight on the same planet, and therefore, these terms are often used interchangeably on planet Earth. However, they are not the same; If an object on Earth has the same mass as an object on the moon, their weights will be different, because the gravity on the moon is less than on Earth.

Weight = mass x gravity

Weight is a force, not a property of an object. Therefore, weight is, within a physics context, often expressed in newtons. However, in the everyday world, "weight" is often measured in units of mass.

## Density

Density is equal to mass divided by volume.

Density is usually measured in units of mass per units of volume. For example, some material might be said to have a density of "one kilogram per litre".

## Momentum

Momentum = mass x velocity

Momentum is related to--but not the same as--inertia. Momentum is something that only moving objects have. If something is not moving, it has inertia, but since its velocity is zero, it has zero momentum (since mass times zero equals zero).

## Impulse

An impulse is a change in momentum.

Impulse = force x time

## Work, energy, and power

Energy is something a system has. Work is something a system does.

Work = force x distance

The SI unit of energy is the joule.

Power is a rate of energy. The SI unit of power is the watt, which is a rate of one joule per second.

## Tension and compression

Tension is what a rope experiences when it's holding something up. Tension is a force that tries to pull something apart. Compression is the opposite: A force that tries to push something inward.

Tension and compression are both most obviously observed in springs. When you stretch a spring, it's under tension. When you press a spring so it gets shorter, the spring is under compression. However, these same forces are at work if you try to pull apart (for example) a rock. If you pull on opposite sides of a rock, the rock is under tension. The force of tension might not be strong enough to tear the rock apart, but the force is still there. Similarly, if you sit on the rock, the rock experiences compression.

## Hooke's Law

Hooke's Law is a very simple and intuitive law. It simply states that when a force acts upon an elastic object (like a spring), the amount which the object deforms (for example, the length by which the spring stretches) is directly proportional to the force applied.

For example, if you apply 2 newtons of force to stretch a spring, and the spring stretches 5 cm, then the same spring will stretch 10 cm if you stretch it with 4 newtons of force.

## Waves

Waves are an important concept in physics. One interesting thing about waves is that you might not realize just how many things in the world are waves until you think about it for a while. The first thing that comes to mind is probably sound, and sound is, indeed, nothing more than waves. Similarly, light takes the form of waves. Many forms of mechanical motion also create mechanical waves.

There are two types of wave: Transverse and longitudinal. Transverse waves are the kind of wave that people traditionally think of when they think of waves; a transverse wave is a wave in which matter moves in a direction perpendicular to the direction of the wave's travel. For example, if you tie a rope to a pole and start shaking the rope back and forth rapidly, the rope curves into a classic wavy pattern. The wave is actually travelling along the length of the rope; however, the rope is moving from side to side, which is perpendicular to the direction of the wave's motion. Therefore, this is a transverse wave. In contrast, longitudinal waves are waves in which matter moves back and forth in the same direction as the wave's direction of travel. Sound is a longitudinal wave. This might be surprising at first, since sound is graphed in a form that looks like a transverse wave. However, it is graphed this way because it makes it easier for humans to understand and analyze what's happening with a sound; in reality, the sound doesn't look like what an audio waveform looks like on an oscilloscope.

## Interference: Constructive and Destructive

Interference is a concept that applies to waves. When people think of "interference", they usually think of two signals that are competing for the same signal space, and which consequently garble each other. However, in physics, interference is divided into constructive interference, and destructive interference.

The difference between the two types of interference has to do with the polarity of waves. Waves, by nature, have a back-and-forth motion: First they move in one direction, then they turn around and head in the opposite direction. If two waves meet while they're heading in the same direction, they will actually amplify each other. For example, if two water waves come together while both of them are pushing water upward, they will come together and create an even bigger wave moving upward. This is constructive interference.

In contrast, destructive interference is when two waves have opposite polarity; one wave simply fills in the other one, and the result is cancellation, so that both waves are muted. Destructive interference is a hugely important topic in audio. If two speakers are out of phase with each other (that is, when one speaker is pushing air forward, the other speaker is pulling air back), they create audio waves that cancel each other out. It is possible, in such a scenario, to have two speakers which are playing quite loud, but which become almost silent when you place them next to each other! Each speaker's sound waves simply cancel each other out, and the result is no wave motion at all. This effect is used deliberately in noise-cancelling headphones, which are headphones that work kind of like the ear protectors that heavy machinery operators traditionally use, except that instead of simply being heavy mufflers that damp all sound going into a person's ear, noise-cancelling headphones actually create sound waves that are out of phase with the sounds that go into them; in this way, sounds are muted before they reach a person's ear. Noise-cancelling headphones can be tuned to cancel out certain sound frequencies, so that (for example) a jackhammer operator could cancel out most of his jackhammer's sound, while still being able to clearly hear the voices of people nearby.

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