Force, work, power and energy

Law of Conservation of Energy

What causes a ball dropped from a height to move? When an arrow is released from a bow, what causes the arrow to acquire kinetic energy?

Since a ball at a height and an arrow in the stretched string of a bow possess potential energy (by virtue of their position and configuration respectively), they both acquire kinetic energy. Does this mean that the potential energy stored inside the ball and the arrow is converted into kinetic energy?


We observed a number of examples in nature in which a form of energy is converted into another. For example, the chemical energy of food we eat gets converted into muscular energy, which is used to do work such as cycling, walking, lifting a load, etc.

During cycling, the muscular energy of the cyclist is converted into the kinetic energy of the bicycle, thereby causing the cyclist to move forward. One thing to be noted here is that the total energy of the system (Cyclist + Bicycle) remains unchanged, i.e., (Muscular energy + Kinetic energy) remains unchanged during cycling.



Conservation of energy

Energy cannot be created or destroyed. It can only be transformed from one form into another. In other words, the total amount of energy in a system always remains constant.


For example, in a burning candle, the chemical energy stored in the wax is transformed into light and heat energy.

The total energy, before and after the burning of the candle, remains constant.

Simple pendulum

A simple pendulum consists of a bob suspended by a section of string from a fixed support. It works on the principle of alternative transformation of kinetic and potential energy. At any instant, the total energy of the bob remains the same.



When the bob is raised to point A, it has only potential energy. It has no kinetic energy at this point as it is at rest. When the bob is released from this point, its potential energy gradually decreases and its kinetic energy gradually increases, such that it consists of both potential and kinetic energy at the intermediate position B. At point C, the potential energy of the bob is zero because this point passes through the zero level. When it moves to the other side, its potential energy increases and kinetic energy decreases, so that it again consists of both potential and kinetic energy at point D. At the extreme point E, the bob is again at rest. Hence, it has only potential energy, which is the same as the energy possessed by it at point A.

Position
Energy of the bob

A
Potential energy only

B
Both potential and kinetic energy

C
Kinetic energy only

D
Both potential and kinetic energy

E
Potential energy only