Kinetic Energy

A force must be applied on a body to accelerate an object. Work must be done in order to apply a force. The body will move with an unvarying speed after the work has been done due to the energy provided by it. The speed and the mass of the body are factors on which the energy transfer that makes up the kinetic energy depends.

Just like all forms of energy, Kinetic energy too can be transformed into other forms of energy and transmitted among the objects. For instance, a moving car may hit a stationary person and then the kinetic energy of the car gets transferred into the person’s body.

What is Kinetic Energy?

Kinetic energy definition is given as

The energy of an object because of its motion or the energy gained by an object from its state of rest to motion.

Formula of Kinetic Energy

Following is the formula of kinetic energy:

\(KE=\frac{1}{2}mv^{2}\)

Where,

  • KE is the kinetic energy of the object
  • m is the mass of an object
  • v is the velocity of an object

Kinetic energy is an example of scalar quantity which means that the quantity has only magnitude and no direction.

SI Unit of Kinetic Energy

The SI unit of kinetic energy is Joule which is equal to 1 kg.m2.s-2.

The CGS unit of kinetic energy is erg.


What are the Examples of Kinetic Energy?

  • A semi-truck travelling down the road has more kinetic energy than a car travelling at the same speed because the truck’s mass is much more than the car’s.
  • A river flowing at a certain speed comprises kinetic energy as water has certain velocity and mass.
  • The kinetic energy of an asteroid falling towards earth is very large.
  • The kinetic energy of the airplane is more during flight due to large mass and speedy velocity.

What are the Types of Kinetic Energy?

Following are the five types of kinetic energy:

Radiant energy

Radiant energy is a type of kinetic energy as it is always in motion travelling through medium or space. Examples of radiant energy are:

  • Ultraviolet light
  • Gamma rays

Thermal energy

Thermal energy is also known as heat energy which is generated due to quick motion of atoms when they collide with each other. Examples of thermal energy are:

  • Hot springs
  • Heated swimming pool

Sound energy

Sound energy is produced by the vibration of an object. Sound energy travels through the medium but cannot travel in vacuum as there are no particles to act as medium. Examples of sound energy are:

  • Tuning fork
  • Beating drums

Electrical energy

Electrical energy is obtained from the free electrons that are of positive and negative charge. Examples of electrical energy are:

  • Lightning
  • Batteries when in use

Mechanical energy

The sum of kinetic energy and potential energy is known as mechanical energy which can neither be created nor destroyed but can be converted from one form to other. Examples of mechanical energy are:

  • Orbiting of satellites around earth
  • A moving car

Difference between Kinetic Energy and Potential Energy

Kinetic energy Potential energy
Kinetic energy is defined as the energy present in an object from the state of rest to motion Potential energy is defined as the energy contained in an object by the virtue object’s position
Formula used is \(KE=\frac{1}{2}mv^{2}\) Formula used is mgh
Vibrational energy is an example of kinetic energy Gravitational energy is an example of potential energy

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How to Calculate Kinetic Energy?

Simply put, Kinetic energy can be calculated by the basic process of computing the work (W) that is done by a force (F). If the body has a mass of m that was pushed for a distance of d on a surface with a force that’s parallel to it.

\(W=F.d=m.a.d\)

The acceleration in this equation can be substituted by the initial (vi) and final (vf) velocity and the distance. This we get from the kinematic equations of motion.

\(W=m.a.d\\ \\ =m.d.\frac{v_{f}^{2}-v_{i}^{2}}{2d}\\ \\ =m.\frac{v_{f}^{2}-v_{i}^{2}}{2d}\\ \\ =\frac{1}{2}.m.v_{f}^{2}-\frac{1}{2}.m.v_{i}^{2}\)

Kinetic Energy’s (K) basic quantity ½mv2 changes when a particular sum of work is acted upon an object.

\(K.E=\frac{1}{2}mv^{2}\)

The total work that is done on a system is equivalent to the change in kinetic energy. Thus,

\(W_{net}=\Delta K\)

This equation is known as the work-energy theorem and has large applications even if the forces applied vary in magnitude and direction. This factor is an important one in the concept of conservation of energy.

Kinetic Energy Facts – What’s Interesting?

The equation tells us some interesting things about kinetic energy:

  • The velocity of a body when squared is dependent on the kinetic energy. This tells us that when the velocity of a body is doubled the kinetic energy gets quadrupled. If two cars, one travelling at 60mph and the other at 30mph the kinetic energy of the former car is four times that of the latter. Thus, the possibility of death increases by about four times too!
  • Kinetic energy cannot be negative; it must either be zero or positive. Velocity can be negative or positive but kinetic energy is at all times positive.
  • Kinetic energy is not a vector unit. The direction of a projectile does not matter. What matters is the velocity it’s thrown with.

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