Energy

# Kinetic energy

## Definition

Kinetic energy is a form of energy, known as energy of motion. The kinetic energy of an object is that produced by its movements that depend on its mass and speed. Kinetic energy is often abbreviated by the letters “Ec” or “Ek.” The kinetic word is of Greek origin “kinesis” which means “movement”.

Kinetic energy formula

Kinetic energy is represented by the following formula:

Ec = ½ mv²

The kinetic energy is measured in joules (J), the mass in kilograms (kg) and the speed in meters above the second (m / s).

As such, kinetic energy is linked to other concepts in physics such as: work, force, and energy. Kinetic energy can only be called when the object is moving and colliding with another can move causing work and force can be referred to as the ability of one organism to cause harm to another. After successful activation of the body, you can keep your kinetic energy applied to the body except a negative one or work with the magnitude of kinetics to return to your initial state of energy or rest.

## Kinetic energy data

Kinetic energy can come from other energies or be converted into other forms of energy. In the case of a roller coaster, cars reach kinetic energy when they are at the bottom of their race, but this gravitational potential energy is converted when they begin to climb. Another example is through kinetic energy that allows the movement of the propellers to obtain electrical or hydraulic energy through the movement of water.

Kinetic energy is due to William Thomson, better known as Lord Kelvin in 1849. Kinetic energy is not typical of our day, since once there were windmills that were used for many activities, such as the main task to grind the Wheat, as the instruments makes use of kinetic energy.

## Kinetic energy types

There is no appropriate type of kinetic energy, however, each particular approach to physics presents its own perspective on this, for example:

### In classical mechanics:

Kinetic energy is understood according to different reference systems, systems of particles or rotating rigid solids. Each of them represents a specific case with specific calculation formulas and variables to consider.

### In relativistic mechanics:

Mechanics influenced by the Theory of Relativity considers kinetic energy based on two scenarios: the kinetic energy of a particle and that of a rotating solid.

### In quantum mechanics:

The mechanics of atomic particles takes into account the kinetic energy based on quantum particles (smaller than an atom) and rigid solids made up of an infinite number of particles.

## Difference between potential energy and kinetic energy

Kinetic energy (Ec) and potential energy (Ep), added together, form the mechanical energy (Em) of an object or system. However, they differ in that, while the first refers to bodies in motion, the second has to do with the amount of energy accumulated within an object at rest.

That said, potential energy depends on the position of the object or system in relation to the surrounding force field, while kinetics has to do with the movements it makes.

## Examples of kinetic energy

### Throw a ball in the air:

We put force on a ball to launch it into the air, letting it fall due to gravity. In doing so, he will acquire a kinetic energy that, when another player confronts him, he must compensate with work of equal magnitude, if he wants to stop and retain it.

### A roller coaster car:

A classic example: the roller coaster car in an amusement park will present potential energy until it begins to fall, and its speed and mass will give it increasing kinetic energy. The latter will be greater if the car is full than if it is empty (because there is more mass).

### Hit someone on the ground:

If we run towards a friend and jump towards him, the kinetic energy that we gain during the race will overcome the inertia of his body and bring him down. In the fall, both bodies will add joint kinetic energy and finally it will be the ground that interrupts the movement.

An example of kinetic energy

Let’s say we are in astronomy class and we want to bury a ball of paper in the garbage can. After calculating the distances, the force and the trajectory, we will have to apply a certain amount of kinetic energy to the ball to make it go from our hand to the garbage can. That is, we must launch it.

When it leaves our hand, the paper ball will start to accelerate and its energy coefficient will go from zero (when it was still in our hands) to X, depending on the speed it reaches.

In a pumped shot, the ball will reach its highest coefficient of kinetic energy the moment it reaches its highest point. Thereafter, as you ideally begin to descend towards the paper, your kinetic force will begin to decrease as you are pulled in by gravity and transformed into potential energy.

When you reach the bottom of the bin, or the ground, and stop, the coefficient of kinetic energy of the paper ball will return to zero.

If, between the moment we throw the ball and the moment it falls, a classmate decides to intercept it, he must apply a counter force equivalent to that which we apply when throwing the ball. And if, instead of throwing a paper ball, we are throwing a lead ball (with a much greater mass), the work to stop the ball must be greater.

Now that you know more about kinetic energy and its definition, you can look around you and try to identify other examples of this type of energy.

## Kinetic energy formula

To calculate the kinetic energy of bodies, the equation is used:

Ec = mv² / 2; principal kinetic equation

This means that the kinetic energy Ec is equal to the mass of the body m times the square of the velocity v, all divided by 2.

We can deduce that the greater the amount of mass, the greater the energy and that the energy is proportional to the speed multiplied by itself.

Kinetic energy is not a vector. This means that if you throw a ball with a velocity of 5 m / s, the ball will have the same kinetic energy, regardless of whether you throw it left, right, or up.

## Properties of kinetic energy

These are some of the properties of kinetic energy:

1. It is a scalar quantity. Studying motion with the kinetic energy theorem assumes that the magnitudes of energy and work are scalar, unlike Newtonian laws, in which the magnitudes are vectors. This is a fundamental difference with the momentum p = m ⋅ v
2. It is always positive. Mass and velocity squared are always positive.
3. It depends on the speed module, but not on your direction or direction
4. Positive work in the body implies that kinetic energy increases (higher final speed). Negative work in the body means that kinetic energy decreases (lower final speed). As examples we can indicate the force exerted by a horse on a car and the friction force respectively.
5. For a body that is not a point of mass, the kinetic energy can be decomposed into translational kinetic energy and rotational kinetic energy.
6. The principle of inertia can be stated by saying that when external work is zero, the kinetic energy of the body does not change. This is because the kinetic energy of the body is constant if the speed is also constant.

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