ICSE Class 6 Physics Chapter 05 Machines

Machines

You have learnt in your history lessons that man has been making and using tools since ancient times. Early man used tools such as scrapers and knives made of stone, and needles made of bone, shown in Figure 5.1(a). Today, we use tools such as those shown in Figure 5.1(b).

We call the things shown in the pictures tools, and not machines, because we are used to thinking of tools as handy implements that simplify our work, and of machines as far more complex things. However, a machine need not be complex at all.

What Is A Machine?

A machine is a device through which a force applied at one point overcomes resistance at some other point. For example, when you scoop up some sugar with a spoon, you apply some force on its handle. Its other end lifts the sugar. To do this, the spoon has to overcome the force of gravity acting on the sugar. This is the resistance offered by the sugar. Thus, a spoon is a machine. Similarly, a knife too is a machine. You apply some force on its handle, while its blade overcomes the resistance offered by the vegetable you are cutting.

A little thought will tell you that most of the common tools we use are, in fact, machines. The devices that we usually refer to as machines, on the other hand, have a large number of sections or pieces, each of which functions as a machine. A sewing machine or a bicycle, for example, has many sections that are themselves simple machines.

Functions Of Machines

Machines can perform a number of functions, such as (a) changing the point of application of a force, (b) changing the direction in which a force has to be applied, (c) changing the magnitude of the force required, (d) changing the speed of the point of application of a force. The following are some common examples of these functions.

Changing The Point Of Application Of A Force

When something is too hot or cold, it is difficult to hold it. In other words, we cannot apply a force on it directly. Usually, we use tongs in such situations. We also use tongs, tweezers or forceps when we do not want to contaminate something or want to work on something fine or delicate. We apply force at one end of the tongs, while its other end applies force on the object. Thus, the point of application of force shifts from one end of the tongs to the other.

Spanners are used to tighten or loosen nuts and bolts, while screwdrivers are used to work on screws. One of the advantages of these simple machines is that they help us apply force on things located at points that are difficult to reach.

Changing The Direction In Which Force Is Applied

It is more convenient to apply a force in some directions than in others. For instance, it is easier to use our body weight to push something down than to pull the same thing up. Some machines make work easier for us by allowing us to apply a force in a direction that is convenient. A hand pump, for example, has a handle attached to a piston inside it. When we apply a downward force at one end of the handle, its other end applies an upward force to pull the piston inside the pump. If a hand pump did not have a handle, we would have to pull the piston up.

A pulley used to draw water from a well serves a similar purpose. If we used a bucket tied to a rope to draw water directly from a well, we would have to apply an upward force. When we pass the rope over a pulley, however, we have to apply a downward force.

Changing The Magnitude Of The Required Force

It is almost impossible to open the metal cap of a bottle with bare hands as this requires a large force. A bottle opener helps us remove the cap by applying a much smaller force on its handle. Nutcrackers, spanners and (car) jacks also help us the same way. They let us use a much smaller force than we would need to use without these machines. In other words, they magnify the force we apply.

Changing The Speed

When you press the key of a typewriter, a long thin metal arm moves out and its head prints a letter on the paper. The speed of this head is much greater than the speed with which you press the key. Thus, this device increases the speed of the point of application of the force. Similarly, when the wheel attached to the pedals of a bicycle turns, it makes the cogwheel attached to the rear wheel turn much faster. This helps you gain speed.

Types Of Machines

We have discussed many different types of machines. Depending on the basic principles on which they work, they can be divided into different categories. These are (1) lever, (2) inclined plane, (3) wedge, (4) screw, (5) pulley and (6) wheel and axle. Though we have included the inclined plane in our list, it is not really a simple machine, as you will see.

Levers

A lever is a rigid body capable of rotating about a point on itself. A rigid body is a body which does not change its shape when forces act on it. Thus, a metal rod or a stout wooden stick is a rigid body. When such a body acts as a lever, it rotates or tends to rotate about some point on itself. This point is called the fulcrum. The fulcrum of a lever is the point about which it can rotate.

The stick resting on the man's shoulder in Figure 5.6 acts as a lever. It rests on the man's shoulder, and can rotate about this point. Therefore, this is the fulcrum of the lever, usually denoted by the letter F.

The force applied to a lever (or any other machine) is called the effort. The point at which the effort is applied is called the effort point, and is denoted by the letter E. The distance FE, from the fulcrum to the effort point, is called the effort arm of the lever.

The resistance overcome by a lever (or any other machine) is called the load. In Figure 5.6, the weight of the bundle acts downwards on the stick. This is the resistance overcome by the stick, or the load. The point on the lever at which this load acts is called the load point, usually denoted by the letter L. The distance FL, from the fulcrum to the load point, is called the load arm of the lever.

Levers are grouped into three classes or types on the basis of the relative positions of the fulcrum, effort and load, as shown in Figure 5.7. The class of a lever depends only on the constituent (load, fulcrum or effort) located in the middle. If the two constituents on the two sides are interchanged, the class of the lever does not change.

Class-one Levers

In a class-one lever, the fulcrum lies between the load and the effort. The positions of the load and effort with respect to the fulcrum do not matter.

If you look at Figure 5.6 again, you will see at once that the stick acts as a class-one lever, as the fulcrum is in the middle, with the load and the effort on either side. Another common example of a lever of this type is the see-saw. The pivot at the centre is the fulcrum, while the two persons on either side act as the load and effort. The handle of a hand pump is an excellent example of a class-one lever. The pivot about which it moves is the fulcrum, the force applied is the effort, and the force which the piston exerts on its other end is the load. Scissors, pliers and tongs of the type shown in Figure 5.8 are also class-one levers. These are called double levers, since they have two identical levers working together.

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