Force, work, power and energy

Consider the following day-to-day activities: Reading, speaking, singing, writing, thinking etc. We require energy to perform these activities, which we derive from the food we eat. Did you know that actually no work is involved in performing these activities? Do you think that a weightlifter does work while standing with weight over his head, as shown in the given figure?



Even if you push a wall with the maximum force that you can apply, the wall will not move. It will be interesting for you to note that even in this case, you are not doing any work at all! Do you know why?


Work is not done in all the above activities because there is a basic difference between the term work and the term which we use for our daily activities.

Scientifically, work is defined as the work done by a force that causes a displacement in an object.

If you push a book placed on a table with a force, then it will move to a certain distance. Scientifically, we will say that some work has been done on the book. Can you name the force against which work is done?

In this case, work is done against frictional force, which exists between the book and the surface of the table.



If you lift the book to a certain height, then a force is exerted against gravity, which displaces the book to a certain height. Hence, one can say that work is done on the book against the force of gravity.

If you push a trolley full of books, then it will move through a certain distance. In this case, the applied force causes a displacement in the trolley. Do you think any work is done on the trolley?