ICSE Class 6 Physics Chapter 6 Simple Machines

6 Simple Machines

Syllabus

1. Simple machines: The lever - identifying load, effort and fulcrum in the three classes of levers; the inclined plane and the screw; the pulley, wheel and axle, wedge, efficiency of a machine, care of machines.

2. Students may be asked to identify simple levers in a variety of common objects and locate the fulcrum, the load and the effort in each case (E).

Machines

In our daily life, though we come across a number of machines, but we do not identify them as machines. This is due to the fact that most of us think that a machine is a complicated device with a number of moving parts and powered by some type of engine or electricity. But, to say in the real sense, even such simple tools such as a hammer, a bottle opener, a spanner or a screw driver are also called machines. In the language of a physicist, a machine is a device that allows us to do work with less effort than if you do the same job yourself without taking outside help of any kind. You might have to apply a large force in lifting a heavy weight by hand, but you can push the same weight up an inclined plane, such as a ramp, very easily. The amount of work done in both the cases is equal, but the inclined plane allows you to put less effort. Hence, the inclined plane works as a machine.

We need a lot of energy to perform any work. But what is energy? In simple terms, energy is the ability or capacity to do work. A simple machine has no energy of its own and at the same time, it cannot do work by itself. It is only when we perform work upon it that it will perform work in turn, upon some other object. In other words we can say that, a machine can do work only after work has been done on it. Thus we derive at a basic law of machines that the work output of a machine is equal to the work input.

Truly speaking, in practice, the work output of a machine is never equal to the work input. The reason is that, the work done (input) on a machine i.e., energy given to machine is partly utilised in overcoming the force of friction between its different moving parts. Therefore, in practice, the work output of a machine is less than the work input.

The main utilisation of a machine is to lift a greater load by applying lesser force (effort) on the machine.

The ultimate description of a machine is that it is a device which makes work easier to do. Machines have made our lives comfortable and faster. They have changed the quality of our life style. We just cannot think of our existence without machines.

Simple Machines

There are actually, six types of simple machines which we use in our daily life. They are (1) lever, (2) inclined plane, (3) wedge, (4) screw, (5) wheel and axle and (6) pulley.

Suppose you want to open the lid of a container filled with oil. You will not be able to open it with bare hands. So, take a spoon and insert it between the lid and the edge of the tin and then press the other end of the spoon. The lid gets opened. Hence, the spoon is used as a machine to open the lid.

Suppose you wish to move a heavy load from one place to another. You may not find it easy to lift or push this heavy load. But if it is easy to lift or push the same weight up an inclined plane, such as a ramp, very easily. The amount of work done in both the cases is equal, but the inclined plane allows you to put less effort. Hence, the inclined plane works as a machine.

Sometimes, a machine like a pulley is used for changing the direction of the force applied. By doing so the effort can be applied conveniently. For example, while drawing water from a well, a pulley is used so that the effort is applied in the direction of our convenience.

Teacher's Note

When you use a spoon to open a tin or pull a door handle, you are using simple machines every day without thinking about the physics behind them.

Lever

A lever is called the simplest machine. It is a rigid rod which is free to move about a point on which it rests. This point of rest is called fulcrum. The object which has to be moved is called load or resistance. The force applied on the lever to overcome load is called effort. In Fig. 6.3, F shows the position of the fulcrum, E is the effort and L is the load or resistance.

A lever is a simple rigid rod, which is free to rotate about a fixed point called fulcrum.

Principle of Lever

Let us consider a straight rod AB with fulcrum at point F. An effort E, applied at point A of the lever, overcomes a load L at point B. The distance AF, from point A at which the effort is applied to the fulcrum F, is called the effort arm. The distance BF, from point B at which the load acts to the fulcrum F, is called the load arm.

According to the principle of a lever: Load - Load Arm = Effort - Effort Arm

L - LA = E - EA

L/E = EA/LA = M.A.

M.A. = Mechanical advantage of a machine.

The mechanical advantage of a machine is the ratio between the load (to be overcome by the machine) and the effort (applied to it).

Mechanical Advantage (M.A.) = Load (L) / Effort (E)

This relation is known as the law of levers.

From the above definition, it is observed that greater the mechanical advantage, the smaller is the effort required to overcome a certain load.

The mechanical advantage of a lever is also equal to the ratio of the length of its effort arm to the length of its load arm.

Types of Levers

Depending upon the relative positions of the fulcrum, load and effort levers are classified into three different types.

1. Class I Levers

2. Class II Levers

3. Class III Levers

Class I Levers

The lever in which fulcrum is in between load and effort is called Class I lever or lever of the first order.

Examples: A beam balance, a see-saw, a pair of pliers, a pair of scissors, claw hammer etc. are some common examples of Class I levers. In all these cases, the load is on one side while the effort is on the other side of the fulcrum.

When you use a spoon to open the lid of a can, you use it as a first class lever.

Teacher's Note

A seesaw in a playground is the most obvious example of a Class I lever that children use and understand intuitively.

Class II Levers

The lever in which load is in between fulcrum and effort is called Class II lever or lever of the second order. This type of lever is used where lesser effort has to be applied to do the work conveniently.

Examples: A nut cracker, a bottle opener, a wheel barrow, a mango cutter, etc. are few examples of Class II levers.

Class III Levers

The lever in which effort is in between fulcrum and load is called Class III lever or lever of the third order.

Examples: Forceps, sugar tongs, fire tongs, knife and human forearm are some of the examples of Class III Levers.

Teacher's Note

When you pick up food with chopsticks or tweezers, you are using a Class III lever where the effort is between the fulcrum and the load.

Inclined Plane

An inclined plane is a gentle slope that helps us to move a heavy load with less effort. It does not look like a machine at all!

Have you seen people moving barrels or oil drums from the ground on to a lorry? They don't lift them but they push them up a sloping board called a ramp or an inclined plane. It is simply a slope over which a load can be pushed up. It is easier to push a heavy load up a slope to a certain height than to lift it up to that height. In simple words, an inclined plane makes it easier to push up a load.

The lifting of drums with the help of an inclined plane is obviously easier than to lift the drums straight. In this case, the inclined plane is a wooden plank. One end of the plank is kept on the edge of the truck and the other end on the floor. This slanting or sloping plank is called the inclined plane. The effort required to push a load up an inclined plane is less than the load.

If the slope is steeper, greater effort will be required to push the load up. A staircase of a building, a road on a hill station, a flyover on a road are some of the examples of an inclined plane.

A wheel chair can be easily pushed up an inclined plane in a hospital.

A winding mountain road is also an inclined plane. Suppose a very steep road is built directly up the side of a mountain. Buses and cars will not be able to go up on such a steep road. That is why winding roads with gradual slope are built on mountains so that less effort is needed to go up the mountain though the distance to be covered becomes greater.

Teacher's Note

The next time you walk up stairs or a ramp, you are experiencing how an inclined plane makes it easier to gain height than jumping straight up.

Pulley

Pulley is another simple machine which is very commonly used in our daily life for lifting loads.

It is a flat circular disc with a groove in its edge and a rope passing through the groove. It is capable of rotating around a fixed point passing through its central axis called axle. Though it is mostly made of some metal, wooden pulleys are still in use. Pulleys are generally used in workshops and factories to lift heavy load. In villages, we see people drawing water from a well with the help of a pulley. We find that mechanics lift heavy engines using a grooved wheel with a long rope. It consists of a wheel with a groove to hold a rope. The wheel moves around an axle fixed to a support called block. While using a pulley, the load is attached to one end of the rope and the effort is applied at the other end as shown in Fig. 6.16.

Several pulleys can be used together to reduce the force needed to lift a load. Such as arrangement is called a block and tackle. The greater the number of pulleys, the smaller is the force needed to lift the load. Such pulleys are used whenever we need to lift heavy loads such as in factories, farms, garages and godowns.

Types of Pulleys

Single Fixed Pulley

The simplest form of pulley is called the "single fixed pulley". A pulley which has its axis of rotation fixed, is called a fixed pulley. Figure 6.18 shows a single fixed pulley in which a string passes around the grooved rim of the pulley. Such a pulley makes our work easier by changing the direction of force. A downword effort is used to lift the load up.

However, a simple pulley does not reduce the effort required to lift a heavy load. Such a pulley is used to lift water from a well, hoist a flag and to lift bricks and cement by workers at a construction site.

Single Movable Pulley

A pulley whose axis of rotation is not fixed is called a movable pulley. Figure 6.19 shows a single movable pulley attached to a load. A string passes through the grooved rim of the pulley. One end of the string is tied to a hook. Tension acts on both sides of the pulley. The effort balances the tension at the free end.

Teacher's Note

Construction sites use pulleys and block-and-tackle systems to lift heavy materials, demonstrating how simple machines solve real-world problems.

Wheel and Axle

A wheel and axle is another form of simple machine. It is actually a combined structure of a wheel and a rod.

The importance of a wheel is known to everyone. A wheel is a circular object capable of rotating along its axis. When a wheel rotates, the axle also rotates with it. Wheels are often used like a roller to reduce friction. The wheel if used with an axle works as a simple machine.

An axle behaves in a manner similar to a wheel. Therefore, a wheel and axle consists of two wheels of different diameters which are attached together. It may also consist of a large wheel attached to an axle. Common examples of wheel and axle are a steering wheel of a motorcar, bicycle pedal, door knobs, etc. In every type of a wheel and axle, the effort is applied to the bigger wheel and the load is applied to the smaller wheel (axle).

Let us take an example of a screw driver. Imagine, its round handle to be a wheel (as it rotates) and the rod attached to it as an axle (it rotates when the handle is rotated). When a force is applied to the handle, the axle rotates with a greater turning effect of force. Hence, the screw driver pushes the screw with a greater turning effect of force into the wood.

A tap is also an example of wheel and axle arrangement. When we turn the tap, the axle inside the tap turns.

Screw

A screw is a simple machine which appears like a nail with grooves made on its circular curved surface. In other words, a screw is like an inclined plane wound around a rod. It consists of a rod with thread. One end of the rod is made narrow or pointed. This is called the tip of the screw. The other end is made flat which is called the head of the screw. A narrow slit is made in the head of the screw so as to turn the screw with the help of a screw driver. The thread acts like an inclined plane. When the screw makes one complete rotation, the rod advances a distance equal to the space between the two consecutive threads.

It takes less force to insert a screw into wood than to insert a nail into wood. This is because the screw makes it move round and round as it goes in, travelling a longer distance than a nail. Also, because of the grooves a screw holds the wood more firmly than a nail.

A jack also works on the principle of a screw. It is commonly used in lifting automobiles for changing flat tyres.

Wedge

A wedge is a simple machine with two inclined planes put together forming a sharpened edge.

It is used for splitting logs. The thinner the wedge, the easier it is to drive it into a log.

A speed boat has its leading edge shaped like a wedge to cut through water easily.

Teacher's Note

Wedges are everywhere - from the knife you use to cut bread to the axe used to split wood, showing how simple geometry creates mechanical advantage.

Efficiency of a Machine

Experimentally, it is found that no machine is ideal. Whenever we do some work on a machine, some work is used up in overcoming the friction existing between different parts of the machine. Therefore, the amount of work done by the machine is always less than the work done on the machine. The ratio of work done by the machine to the work done on the machine is called its efficiency.

Efficiency = Work done by the machine on the load / Work done on the machine by the effort

The work done by the machine is called the output work (or energy) whereas the work done on the machine is called input work (or energy).

Therefore, the ratio of output energy of the machine to the input energy on the machine is also known as efficiency of the machine. In terms of percentage, we write it as:

Efficiency % = Output energy / Input energy - 100%

If a machine is 80% efficient, it means that 80% of the work input to the machine is obtained as the useful work output. The remaining 20% of the work input is lost in overcoming the friction.

A machine with 100% efficiency is called an ideal machine. No machine has efficiency more than 100%. In fact, the efficiency of every practical machine used by us is always less than 100%.

Every machine has friction acting with in its internal parts. Therefore to overcome this friction some amount of energy is utilised and hence the efficiency of practical machines is always below 100%.

Care of Machines

Machines are a gift of science. They play an important role in our lives. There would have been no economic progress if machines would not have been there. Thus, for their proper functioning and efficiency, we must take proper and timely care for them.

1. Machines should be protected from rust by applying paint or rust-proof coating.

2. Regular oiling of parts increases their efficiency, otherwise they wear out or make too much of noise due to friction.

3. Machines, when not in use, should be kept covered to protect them from dust.

Teacher's Note

Regular maintenance of bicycles, cars, and household appliances demonstrates why caring for machines keeps them working efficiently for longer.

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