Work and Energy

Introduction to Work

Whenever you visit the railway station you must have observed the coolies on the station. The coolie puts down the luggage and pulls on to it to keep it from falling down suddenly. The coolie then pulls up and the luggage is moved down till it touches the earth. What is happening here? We can see that some work is done here. So, what is work? Let us study below.

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What is Work?

Work is the force causing the movement or displacement of an object.It is a scalar product of the force acting on an object and the displacement caused by that force, this happens in the case of a constant force.It has no direction due to the nature of a scalar product even though both force and displacement are vector quantities.

The expression for work depends upon the particular circumstances. For instance, in compressing a gas at a constant temperature the workdone is the product of pressure times the change in volume. It transfers energy to the body hence work done on a body is equal to the increase in the energy of the body. However, f the applied force is opposite to the motion of the object, the work done is considered to be negative, it means that energy is taken from the object. Its SI unit is joule.

No Work is Done in spite of Working Hard

Let us consider an example, wherein a waiter is carrying a tray high above his head, by one arm, and as he is walking at a steady pace across a room, although you might think he’s working hard.

But, scientifically he is not doing any work. The waiter is using force to push the tray above his head, also the tray is moving across the room as the waiter walks. But the waiter’s lifting of the tray does not cause the tray to move. There must be an element of force in the direction of the displacement to cause a displacement.

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Workdone Mathematically

The basic calculation of work is:

W = Fd, where W = Work and F = Force and d = Displacement or the object distance.

For instance,  a player throws a ball with a force of 10 Newtons. The ball travels 40 meters. What is the total work?

W = Fd

Therefore, W = 10 N * 40 meters

Hence: W = 400 joules

Workdone Scientifically

Following are the places where work is done scientifically:

  • Launching a rocket.
  • Internal combustion engine in a vehicle.
  • Drawing water by a pump.
  • The output from the turbine.

Workdone by a Constant Force

When a force is acting on an object over a distance, then work is done on the object. Although physically, the work done on an object is the change in its kinetic energy that the object experiences.

Solved Example For You

Q. When acted upon by a force F, a body suffers a displacement S. The workdone will be negative if the angle between F and S be:

a. An acute angle           b. An obtuse angle

c. always equal to 00    d. Always equal to 900

Sol: b. An obtuse angle

If θ be the angle between force F and displacement SW=F.S.cosθ. So, workdone to be negative, cosθ should be negative.
And, cosθ is negative when θ is an obtuse angle.

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