In physics, work is defined as the transfer of energy resulting from the application of a force over a distance. Energy, on the other hand, is the ability to do work. The relationship between work and energy is tightly connected, as work done on an object results in a change in its energy.

The work-energy theorem states that the work done on an object is equal to the change in its kinetic energy. This theorem allows us to solve problems relating to the transfer of energy in various scenarios. For example, when a force is applied to push an object over a specific distance, the work done can be calculated by multiplying the force applied by the distance traveled.

Let's consider an example: lifting a book from the ground to a shelf. The force exerted to lift the book against gravity does work, and this work results in an increase in the book's gravitational potential energy. Similarly, if the book is dropped from the shelf, it gains kinetic energy as it falls under the influence of gravity.

To calculate the work done in such cases, one must consider the force applied, the distance over which the force is applied, and the angle between the force and the displacement. It's important to note that work is a scalar quantity and is measured in joules (J).