What is work?

We use the term ‘work’ in various manners in our daily life. A gatekeeper guarding a house all day long, sitting on a tool, may claim to have done a lot of work, but in the words of physics that is not any work. In terms of physics the word ‘work’ is well defined. If a force F is applied on any object and the object traverses a distances during the application of force (that is displacement occurs) then, the amount of work W done by the force is: W=Fs
The product of applied force on a body and its displacement along the direction of force applied is called work done.

Unit of work

Work’s unit = F‘s unit × s‘s unit
=Nm
= Joule (J)

What is 1 joule?

The SI unit of work is Joule. if a force 1N is applied to a object and it moves 1 m direction of the force then the work done is said to be 1 Joule.

Dimension of work

[W] = ML²T⁻²

Force is a vector quantity and the distance traversed or displacement is also a vector quantity but in case of work the product of those two vectors is a scalar quantity.

As individual vectors there is no such condition that the direction of force and distance travelled has to be same but in these book, we will only discuss the force and distance travelled in the same direction.

Have you noticed, while discussing work, we said that the force’ has done the work. A person or a machine may push any object to some distance by applying a force. In terms of daily life we say that the person or the machine has done the work. But in physics, neither the person nor the machine does the work, always the force applied does the work. This force may be applied by a person or a machine.

Let us assume that, you have pushed an object to a distance s by applying a force F and left it after setting it in motion. The object has travelled an additional distance d and at last it has stopped. How much work is done?

The amount of work done is W=Fs, as no force was applied while traversing the distance d, so no work is done.

Question: Your mass is 50 kg. You have climbed up a 10 storied building, how much work have you done? (Height of each floor is 3 m)

Answer: If your mass is 50 kg,

weight is = 50 × 9.8 = 490 N.

This weight is a force, that is acting downwards. If you want to climb up, you have to exert an equal force upwards to draw yourself up. So, upward applied force,
F =490 N

Distance travelled upwards: 10 × 3 m= 30 m

So the amount of work done, 490 N × 30 m = 14700 J = 14.7 kJ

Suppose, a moving object is coming towards you, you try to stop the object by applying force F, the object pushes you backwards by a distance s. How much work was done by the force applied by you? Certainly you have noticed that, this time the distance traversed is not along the direction of force, rather opposite to it. So the amount of work done

W=F (-s)=-Fs

That is, the work done is negative. In our daily lives, we talk about work and useless work, but what is meant by positive and negative work in terms of physics? If the work is positive, then it can be said that work is done by the force. If it is negative, then it is said that the work is done ‘on’ the force or against the force. We must have a clear conception about energy before understanding what has just been written.