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Kinetic Energy: The Force That Keeps The World Moving

From the motion of the distant stars to the vibration of tiny electrons, kinetic energy plays a significant role in shaping the world around us.

Kinetic Energy: The Force That Keeps The World Moving
A long exposure shot of some amusement park rides. Jason Chen/Unsplash

The world is moving fast but did you ever wonder how to measure the energy of a moving object?

To accelerate an object requires the application of force. And the application of force is, quite literally, work. When work is done on an object, energy is transferred. The energy that is transferred is known as kinetic energy and it depends on the mass and speed of the moving object.

Kinetic energy is the measure of work done by any object when it moves, it is defined in terms of mass (m) and velocity (v). The motion of an object can be horizontal, vertical, elliptical, etc., but kinetic energy applies in every case where there is motion. 

How to measure kinetic energy?

Kinetic Energy: The Force That Keeps The World Moving
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Kinetic energy is a scalar quantity. This means that it is independent of the direction of an object, and is only described in terms of magnitude. The equation for kinetic energy is:

 

      KE = ½ mv2     

We can see from this that, when the mass (m) of an object increases, so does its kinetic energy, and when the velocity (v) of an object is doubled, the value of kinetic energy increased by four times. Therefore, the kinetic energy is directly proportional to an object's mass and velocity.

The SI (International System of Units) unit of Kinetic energy is Joule (Kg.m2.s-2 and in CGS (centimeter–gram–second system), it is defined in terms of erg (10-7 Joule or gm.cm2.s-2)

  • Kinetic energy equation

The work-energy theorem states that the total work done on a system is equal to the change in kinetic energy of the system.

Wtotal = ΔK

Work is equal to the change in kinetic energy (Δ K). When a force (F) is applied to an object of mass (m) then the work (W) done to move the object by a distance (d) on the surface is given as:

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W=F * d where

F = mass (m) * acceleration (a) so

W = m * a * d

To derive the kinetic energy equation, we also the kinematic equation, 

                                                v2  = u2  + 2a *Δd where  

(u) is the initial velocity, (v) is the final (v) velocity and (Δd) is the displacement. Solving for acceleration gives us:

                                                    a = F/m and

                                                F/m = (v2 - u2)/2Δd

                                                F * 2Δd = m (v2 - u2)

                                                F *Δd = m (v2 - u2) / 2 

Since an object at rest has zero velocity, this can be simplified to:

                                                F *Δd =  mv2/ 2 

                                                  F *Δd = ½ mv2

Since F *Δd is the equation for work, and work is equal to the change in kinetic energy (ΔK), we have:

                                                    W = ½ mv2 

Types of kinetic energy

Kinetic Energy: The Force That Keeps The World Moving
Source: Iván Rivero/pexels

Depending on the parameter taken into account, there are two ways in which kinetic energy can be categorized:

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1. On the basis of movement

Objects show different kinds of motion, from vibrating quantum particles to the large rotating turbines.

  • Translational kinetic energy

This is the energy due to the movement of a rigid body in a straight line Examples include the motion of a bullet, an apple falling from a tree, etc.

  • Vibrational kinetic energy 

Kinetic Energy: The Force That Keeps The World Moving
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Vibrational kinetic energy is simply the kinetic energy of an object due to its vibrational motion. The vibration of a cell phone in your pocket, or a drum when it is hit by a hand or stick are both examples of vibrational kinetic energy.

  • Rotational kinetic energy

Rotational kinetic energy or angular kinetic energy is kinetic energy due to the rotation of an object, and is a part of its total kinetic energy. Rotational kinetic energy can be expressed as: Erotational=12Iω2">Erotational= ½ I ω2 where (ω"> is the angular velocity, (I">is the moment of inertia around the axis of rotation, and (E) is kinetic energy.

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Common examples of this type of KE is the rotation of the Earth, and the movement of a flywheel. 

2. On the basis of energy 

The nature of kinetic energy also depends on the amount and type of energy in the system, and the effect this has on the motion of particles in the system.

  • Thermal energy

The thermal kinetic energy of an object or system is that part of its internal energy that is responsible for the temperature of the system and is involved in heat transfer. Thermal energy is generated due to the motion of atoms when they collide with each other. Examples of thermal energy are: The movement of heated water in a swimming pool or hot water springs is an example of thermal kinetic energy.

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  • Electrical Energy

Kinetic Energy: The Force That Keeps The World Moving
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An electric current is a form of energy that results from the motion of free electrons. Lightning and light bulbs in use are examples of electric energy in motion.

  • Radiant Energy

Radiation can be defined as small (subatomic) particles with kinetic energy that are radiated or transmitted through space. Examples of radiant energy include ultraviolet light, gamma rays, or the heat from a fireplace when it warms a room. Absorption in nuclear reactions and particulate radiation is a process of taking up kinetic energy of particles or the combination of particles with an atom, a nucleus, or another particle.

  • Sound energy

Sound is a form of vibrational kinetic energy that can be heard. Sonic kinetic energy produces moving energy by using longitudinal waves. Common examples are music, speech, etc. In a vacuum, there is no sound, because there is no medium to transmit the vibrations.

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What is the relationship between potential and kinetic energy?

Kinetic Energy: The Force That Keeps The World Moving
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Potential energy is the energy possessed by an object, or body, by virtue of its position relative to others. It can also be produced due to internal stresses, electric charge, and other factors. It is a measure of how much stored energy an object has.

The change in the potential energy of an object can also be considered in different configurations. For example, the gravitational potential energy of a ball will change depending on how far it is from the center of the Earth — a ball that is on top of a tall mountain has more potential energy than one that is two feet off sea level.   

Other examples of systems which store potential energy include electrically charged particle near or far from another charge, and a rubber ball which can be squashed or stretched.

The potential energy (U) of a body at some point ( is defined as the work done on the object by an extra, imposed force to move it from a reference position to its current position. This reference point is called the "zero point" of potential energy.

There are a number of different types of potential energy, including gravitational potential energy (in a uniform field or due to two point masses, electrical potential energy  (due to a point charge or in a uniform field), magnetic potential energy, and the potential energy stored in a stretched or compressed spring.

Interesting facts about kinetic energy

Kinetic Energy: The Force That Keeps The World Moving
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Kinetic energy plays a role in all types of movement.

  • Kinetic energy is being used in the development of various types of alternative and renewable energy-based innovations, the movement of bike pedals can be used to supply energy for headlights, windmills generate wind power through movement, hydroelectric power plants harness energy from moving water to generate electricity. Another such innovative invention is the kinetic dance floor that stores energy from the footsteps that fall on its surface and use that energy to produce electricity.
  • Scientists are also looking for ways to store the kinetic energy from the vibrations in home appliances and devices such as dishwashers, juicers, grinders, smartphones, washing machines, mp3 players, etc.
  • The movement of water in the river constantly creates the energy of motion, but when a dam is created to prevent the flow of the river, then this kinetic energy is both used to generate electricity by doing work, and is stored in the form of potential energy in a reservoir. 

  • The word ‘kinetic’ originated from the Greek word ‘kinesis’ which means motion, and the origin of the two energy forms (potential and kinetic) is also found mentioned in the principles of actuality and potentiality propounded by the great Greek philosopher Aristotle. However, the term ‘kinetic energy’ was first coined by English physicist Baron Kelvin Sir William Thomson, in 1850, and a couple of years later, Scottish engineer William Rankine introduced the word ‘potential energy’.
  • Recently, an aerospace engineer named Tom Stanton performed a unique experiment with his bicycle. He developed a flywheel-based kinetic energy recovery system, attached to his bike. This innovation can have numerous applications when it comes to energy storage. 
  • Roller coasters are both fun and scary, and the crazy experience you get on them is the result of a transfer between potential energy and kinetic energy. When you are at the maximum height in a roller coaster, you achieve maximum potential energy (in addition to fear) and when moving towards the bottom, it’s the kinetic energy that brings all the thrill. 

Every day we experience kinetic energy in many ways and it is almost impossible to miss the impact that this movement-driven phenomenon has on our lives.

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