Forces and Machines
We use force when we do work. We use force when we write on a paper, open a door, pedal a bicycle or push an iron nail into the wood. We cannot see a force but we can see its effects.
Friction
What happens when we stop pedalling a bicycle? Why a rolling ball stops after moving some distance on the ground? There is a force which opposes the motion of the bicycle and the rolling ball.
A force which slows down and stops the things from moving is called frictional force or friction. Friction appears when a moving object is in contact with another. It is a contact force. There is less friction when smooth surfaces such as glass and a marble slide over each other. There is more friction when rough surfaces such as sand paper and bricks slide over each other.
Activity
Place equal sized wooden and ice blocks side by side on a table. Push them to slide over the surface of a table as shown in the figure.
Which of the two blocks needs lesser force to slide over the table and why?
Roughness of the surfaces increases friction. No surface is perfectly smooth. A surface that appears smooth has roughness also when seen under a microscope.
Some materials are deformed (deshaped) under pressure. Deformed materials also cause friction. So, roughness of surfaces and deformations are the causes to increase the friction. Objects moving in liquids and gases also face friction. Water resistance is a force which slows the objects moving through water. Air resistance is a force which slows the objects moving through air.
Advantages And Disadvantages Of Friction
Advantages of Friction
Many of our daily life activities such as walking, writing and stopping the fast moving vehicles by applying brakes are due to friction. We cannot write if there is no friction between paper and the pencil. When we write and rub a pencil on the paper, friction is produced due to which carbon particles leave a mark on the paper. Friction between our shoes and the Earth enables us to walk or run on the ground. When the grooves on the sole of our shoes slide on rough surface of the floor, friction is produced. It gives our shoes the grip we need to walk.
Similarly, friction between the tyres of the vehicles and the ground enables them to stop when brakes are applied. Birds cannot fly if there is no air resistance. The reaction of pushed air enables the birds to fly.
Disadvantages of Friction
Despite friction is very important to us, it has many disadvantages too. Friction offers resistance to a moving object at high speed and reduces the speed of moving objects.
We know that worn out tyres of the vehicles are replaced after some period. What is the cause of wear and tear of tyres? The answer is friction. Similarly, if we do not grease the chain of a bicycle, the chain and the pulley will wear out soon due to friction.
The moving parts of engines and machines which rub against each other become very hot. This increases the wastage of energy. The sole of our shoes is worn out due to friction with the ground.
Methods to Reduce Friction
Friction can be reduced by the following methods
Polishing of surfaces
Polishing of surfaces of the objects reduces the friction.
Using lubricants
Use of lubricant (oil or grease) between the moving parts of machines can reduce friction.
Polishing of surfaces of the objects reduces the friction.
Use of lubricant (oil or grease) between the moving parts of machines can reduce friction.
Using rollers or wheels
Friction in rolling an object over another is much less as compared to sliding. Therefore, rollers or wheels are used to pull a cart or a TV trolley on the floor. They reduce friction and make our work easier.
Using ball bearings
Ball bearings change sliding friction into rolling friction. They are usually placed around an axle, so the rotation of the wheel becomes easier.
Streamlining the bodies
Cyclists use very narrow and hard tyres in their bicycles to reduce friction with the road. They wear tight dress and bend their bodies to give themselves a streamline shape which moves through the air easily. This shape faces minimum air friction when moving. For example, sports cars are always streamlined so that they can move fast. Similarly, the swimmers also acquire streamlined shape to reduce the water friction. Aeroplanes, ships and boats have streamlined shapes and they face less friction to move.
Gravitational Force
We know that when a ball is thrown upward it comes back to the Earth. It is easy to go down Stairs as compared to moving upstairs. A cricket ball thrown upward, comes downwards. Similarly, an apple or an orange from a tree always falls downward. Why does everything fall down? Actually, Earth attracts all the bodies towards itself. This force of attraction is called gravitational force (force of gravity). Gravitational force not only exists between the Earth and other bodies but also exists between all the objects around us. It is a pulling force. It depends upon the mass of the objects and the distance between their centres. The greater the mass of an object, the greater will be gravitational force. The larger the distance between the centres of the objects, the smaller will be the gravitational force. Gravitational force between small objects is too small to be noticed. It can easily be observed with massive objects such as the Sun, the Earth and the other planets. Gravitational force of the Sun pulls the Earth and other planets to orbit around it. It is the gravitational force of the Earth that:
(i) holds us on the surface of the Earth and stops us floating away into the space.
(ii) keeps the moon revolving around the Earth.
(iii) makes rain fall on the Earth.
(iv) makes the rivers flow downstream.
Lever
A lever is a simple machine which turns about a fixed point. When a force is applied at one end of the lever, it turns about the fixed point to lift the load at the other end.
The fixed point about which a lever turns is called its fulcrum (F). The force which is applied on the lever is called effort (E) and the weight which is lifted is called load (L). Scissors, pliers, claw hammer, door, and staplers, etc., are the examples of the levers.
There are three kinds of lever based on relative positions of the fulcrum (F), the effort (E) and the load (L).
Using ball bearings
In the first kind of lever, the fulcrum is between the effort and the load.
First kind of levers can produce large force from a small effort. In this case the effort arm which is the distance between effort and fulcrum, is long whereas the load arm, the distance between load and fulcrum is short. By a longer effort arm, greater force will be produced. Scissors, pliers, lid opener, and claw hammer, etc., are the examples of the first kind of lever.
Second kind of lever
In the second kind of lever, the load is between the fulcrum and the effort.
Bottle opener, wheelbarrow, nut cracker, paper cutter and door, etc., are the examples of second kind of lever.
Third kind of levers
In the third kind of lever, the effort is between the fulcrum and the load
In this case the effort moves a short distance, whereas the load moves a long distance.
Pair of tongs or forceps, broom, and the human arm, etc., are the examples of third kind of lever.