Kinetic and Potential Energy

Kinetic Energy
Potential Energy

Kinetic energy is energy possessed by a body by virtue of its movement. Potential energy is the energy possessed by a body by virtue of its position or state. While kinetic energy of an object is relative to the state of other objects in its environment, potential energy is completely independent of its environment. Hence the acceleration of an object is not evident in the movement of one object, where other objects in the same environment are also in motion. For example, a bullet whizzing past a person who is standing possesses kinetic energy, but the bullet has no kinetic energy with respect to a train moving alongside.

Comparison chart

Kinetic Energy

Potential Energy

Definition The energy of a body or a system with respect to the motion of the body or of the particles in the system. Potential Energy is the stored energy in an object or system because of its position or configuration.
Relation to environment Kinetic energy of an object is relative to other moving and stationary objects in its immediate environment. Potential energy is not relative to the environment of an object.
Transferability Kinetic energy can be transferred from one moving object to another, say, in collisions. Potential energy cannot be transferred.
Examples Flowing water, such as when falling from a waterfall. Water at the top of a waterfall, before the precipice.
SI Unit Joule (J) Joule (J)
Determining factors Speed/velocity and mass Height or distance and mass

Contents: Kinetic and Potential Energy

Interconversion of Kinetic and Potential Energy

Law of conservation of energy states that energy cannot be destroyed but can only be transformed form one form into another. Take a classic example of a simple pendulum. As the pendulum swings the suspended body moves higher and due to its position potential energy increases and reaches a maximum at the top. As the pendulum begins its downward swing, the stored potential energy is converted into kinetic energy.

When a spring is stretched to one side, it exerts a force to the other side so it can come back to its original state. This force is called restoring force and acts to bring objects and systems to their low energy level position. The force required to stretch the spring is stored in the metal as potential energy. When the spring is released, the stored potential energy is converted into kinetic energy by the restoring force.

When any mass is lifted, the gravitational force of the earth (and the restoring force in this case) acts to bring it back down. The energy needed to lift up the mass is stored as potential energy due to its position. As the mass is dropped, stored potential energy is converted to kinetic energy.

Etymology

The word 'kinetic' is derived from the Greek word κίνηση or 'kinesis' which means 'motion'. The terms 'kinetic energy' and 'work' as understood and used today have their origin in the 19th century. The word kinetic energy is believed to have been coined by William Thomson alias Lord Kelvin.

The term "potential energy" was coined by William Rankine who was a Scottish physicist and engineer in the 19th century.

Types of Kinetic Energy and Potential Energy

Kinetic energy can be classified into two types, depending on the type of objects:

Rigid non rotating bodies have rectilinear motion. Thus translational kinetic energy is kinetic energy possessed by an object moving in a straight line. Kinetic energy of an object is related to its momentum (product of mass and velocity, p= mv where m is mass and v is velocity). Kinetic energy is related to momentum through the relation E = p^2 / 2m and hence translational kinetic energy is calculated as E = ½ mv^2. Rigid bodies which rotate along their center of mass possess rotational kinetic energy. Rotational kinetic energy of a rotating body is calculated as the total kinetic energy of its different moving parts. Bodies at rest also have kinetic energy. The atoms and molecules in it are in constant motion. The kinetic energy of such a body is the measure of its temperature.


Potential energy is classified depending on the restoring force applicable.

Example, when a book is placed on top of a table, energy required to raise the book from the floor and energy possessed by the book due to its elevated position on the table is gravitational potential energy. Here gravity is the restoring force.

Applications

References

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