Kinetic energy is the energy of motion: if something is moving, it is said to have kinetic energy. According to classical mechanics, the kinetic energy (E) of a non-rotating object depends on its mass (m) and velocity (v).
E = ½mv2
Since energy is a scalar quantity, it doesn’t depend on the direction and is always positive. If you double the mass, you double the energy. However, if you double the velocity, energy will increase by a factor of four.
Kinetic energy can be categorized into three groups based on the type of motion of the object.
- Translational Kinetic Energy: is the energy due to motion from one position to another. For example, a train moving on a track or objects freely falling due to gravity have translational kinetic energy.
- Rotational Kinetic Energy: is the energy due to rotational motion. Earth’s rotation is a perfect example of rotational kinetic energy.
- Vibrational Kinetic Energy is the energy due to vibrational motion. The motion of tunning fork is a prominent example of vibrational kinetic energy.
The imperial unit of kinetic energy is foot-pound, while the standard unit is Joule. It can be transferred between objects and converted into other types of energy.
For example, a runner uses chemical energy (provided by food) to accelerate himself. In this case, the chemical energy has been transformed into the energy of motion, i.e., kinetic energy. However, the process is not fully efficient, as a lot of energy is wasted in heat.
Kinetic energy mainly occurs in five different forms: mechanical, electrical, thermal, radiant, and sound. To better explain this quantitative property, we have gathered a few simple and most basic examples of kinetic energy that happen in everyday life.
Table of Contents
1. Moving Vehicle
Image credit: GIPHY
A Form of Mechanical Energy
The very definition of kinetic energy is the energy that a body possesses by virtue of being in motion. By this definition, every moving vehicle has a specific kinetic energy.
The more the mass and velocity of a vehicle, the more kinetic energy it will have. A car will have higher kinetic energy than a motorcycle (considering both are moving at the same speed, but the car has more mass).
Similarly, a flying fighter jet or a spacecraft (such as International Space Station in Low Earth Orbit) has a very high amount of kinetic energy.
2. Cycling
A Form of Mechanical Energy
Bicycling is a rich source of kinetic energy. The cyclist initially has chemical energy stored in his body as a result of food intake. As he applies a downward force upon the bike pedal, the chemical energy is converted into kinetic energy.
However, this energy conversion is not very efficient. The cyclist also uses a significant amount of chemical energy to produce heat and to overcome friction and air resistance.
3. Dropping A Phone On Floor
A Form of Mechanical Energy
What happens when you accidentally drop your phone? It accelerates due to the gravitational force, gaining speed and momentum.
Any falling object would continue accelerating until the upward force of air resistance completely balances the downward force acting due to gravity. In this case, however, we can neglect the air resistance, since it is much lower than the gravitational force.
Initially, at the highest point, the phone possesses maximum potential energy. As it falls, this energy is transformed into kinetic energy. The more the mass of the phone, the more kinetic energy it will achieve.
When the phone hits the floor, this kinetic energy goes into producing sound, causing the phone to bounce, and breaking or deforming its body.
4. Bullet Fired From A Gun
A Form of Mechanical Energy
The bullet flying through the air has an extremely high kinetic energy. It is also called muzzle energy. If we don’t take external factors into account (such as gravity and aerodynamics), muzzle energy roughly indicates the destructive potential of a given firearm or cartridge.
The faster the bullet moves and the heavier it is, the higher its kinetic energy and the more damage it will do.
5. Lightning During A Thunderstorm
A Form of Electrical Energy
Electrical energy is a type of kinetic energy caused by the flow of negatively-charged electrons. The amount of energy is proportional to the speed of electrons: the quicker they move, the more energy they carry. This very movement of electrons is what powers our electrical devices.
Lightning during a thunderstorm is a prominent example of electrical energy. What you actually see is the instant discharge of electrons caused by static electricity in clouds. As lightning heats the air, it produces a shock wave, causing the sound of thunder.
6. Electricity Provided By A Car Battery
A Form of Electrical Energy
A car battery converts chemical energy into electrical energy, which can be accessed via the battery terminals. The chemical process in the discharging battery releases the electrons from anode to cathode. These moving electrons provide electricity to the circuits in the car.
To charge the battery, the flow of electrons is reversed (from cathode to anode). Moreover, these rechargeable batteries are designed to release a high burst of current and then be quickly recharged.
7. Vibrating Stereo Speakers
A Form of Sound Energy
Sound is the movement of energy through a medium (such as water or air) and is caused by vibrations. Sound energy travels in the form of waves and reaches our eardrums, which then vibrate, and our brain interprets it as sound.
Stereo speakers (or anything that produces sound) works in the same way. If you play it louder and place your hand on it, you will feel it vibrate. What’s actually happening here is the speaker is moving back and forth, pushing on air particles, which changes the air pressure and generates soundwaves.
Another great example would be playing drums; when you strike a drum, its surface vibrates and causes sound.
Unlike light, sound cannot travel through a vacuum, as there are no atoms to transfer vibrations.
8. Photons Emitted By Incandescent Light Bulb
A Form of Radiant Energy
In a traditional light bulb, also called an incandescent light bulb, the electric current travels from one metal contact to another. As the current flows through the wires and tungsten filament, the filament heats up to the point where it starts emitting photons, small packets of visible light.
The bulb also produces a lot of heat in addition to light. A 60 watt light bulb, for example, converts 60 joules of electrical energy per second into light and heat energy — both are forms of radiant energy.
Radiant energy refers to the energy that travels by particles or waves. It is generated through electromagnetic waves, which we usually experience in the form of heat.
Read: Who Invented The Light Bulb? The Complete Truth
9. Radio Waves Travelling At Speed Of Light
A tower emitting radio signals
A Form of Radiant Energy
Radio waves also travel in the form of waves. They have frequencies ranging from 3 kHz to 300 GHz, and corresponding wavelengths of 100 kilometers and 1 millimeter. Like other electromagnetic waves, radio waves travel at the speed of light. Radio stations use these waves to transmit their content across vast distances.
The other good example of radiant energy is rays coming from the Sun. This is why you feel hotter in the sunlight than in the shade.
10. Boiling Water
A Form of Thermal Energy
Like radiant energy, thermal energy can be experienced in the form of warmth or heat. However, there is a big difference between these two: while radiant energy describes the motions of particles or waves, thermal energy refers to the level of activity among molecules and atoms in an object.
When atoms and molecules move faster and collide with each other, they create thermal energy. Because of this motion, thermal energy is considered a form of kinetic energy.
Boiling water is the best way to visualize thermal energy. When you heat the water, the kinetic energy of individual water molecules increases. And it keeps on increasing with temperature until the water reaches the boiling point.
Read: What Is A Particle Accelerator? How Does It Work?
The geothermal energy that comes from the volcanic action of Earth and the decay of natural minerals is also an example of kinetic energy.
