LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 11 : FORCE AND PRESSURE

FORCE

 

·         A push or pull on an object is called force.   

·         Forces are used in our everyday actions like pushing, pulling, lifting, stretching, twisting and pressing.

 

Force is due to an interaction:

 

·         When two objects interact with each other, they can exert forces on each other. These forces can either attract or repel the objects, depending on the nature of the interaction.

 

Force has magnitude as well as direction:

 

·         The strength of a force is expressed by its magnitude. 

·         The magnitude of force is expressed in the the SI unit of force called Newton. Its symbol is N.

·         1 Newton is the force which can make an object of 1 kg mass to move an acceleration of 1 m/s².

·         When two forces act along the same direction, then the net force acting on the object is equal to the sum of the two forces and when they act in opposite direction, and then the net force acting on the object is equal to the difference between the two forces. 

·         If the two forces applied to an object are equal in magnitude and act in opposite directions, then the net force acting on the object is zero.

 

EFFECTS OF FORCE:

 

A force can produce the following effects.

·         A force can move a stationary object.

·         A force can stop a moving object.

·         A force can change the speed of a moving object. 

·         A force can change the direction of a moving object. 

·         A force can change the shape and size of an object.

 

TYPES OF FORCES

The most common types forces are:

·         Muscular force

·         Frictional force

·         Magnetic force

·         Electrostatic force and

·         Gravitational force.

 

CONTACT FORCES

A force which can be exerted bi an object on another object only through physical touching or contact is called a contact force. Examples of contact forces are

·         Muscular force and

·         Frictional force

 

MUSCULAR FORCE

The force exerted by the muscles of the body is called muscular force.

As the muscular force can be applied to an object only when our body or body of an animal is in contact with the object, therefore, muscular force is a contact force.

 

FRICTIONAL FORCE OR FRICTION

The force which always opposes the motion of one body over another body is called frictional force or friction. 

Since frictional force arises only when the surfaces of two objects are in touch with each other, frictional force is an example of of contact force. 

NON CONTACT FORCES

A force which can be exerted bi an object on another object even from a distance without touching each other is called a non contact force.

The examples of non contact forces are

·         Magnetic force

·         Electrostatic force and

·         Gravitational force.

 

MAGNETIC FORCE

The force exerted by a magnet is called magnetic force. 

Since a magnet can exert its magnetic force on iron objects from a distance even without touching them, therefore, magnetic force is a non contact force. The magnetic force between a magnet and an iron object is always that of attraction. A magnet can attract or repel another magnet. 

 

A magnet has two poles, North Pole and South Pole. 

 

There is a magnetic force of repulsion between the like poles of two magnets and there is a magnetic force of attraction between the unlike poles of two magnets. 

 

The magnetic force is widely used in our everyday life. 

 

ELECTROSTATIC FORCE

There are two types of electric charges. They are Positive electric charges and Negative electric charges. 

 

The force exerted by an electrically charged object is called electrostatic force. As an example, when a plastic comb is rubbed in dry hair, it gets electric charges and this electrically charged comb can attract tiny pieces of paper. 

 

Electrostatic forces may be attractive or repulsive. Electrostatic force between two like charges is repulsive and between two unlike charges is attractive. Therefore we can say, a positive charge repel another positive charge but attracts a negative charge. 

 

The electrostatic force can be exerted by a charged object on another object from a distance even when they are not in touch with each other; therefore, electrostatic force is an example of non contact forces. 

 

GRAVITATIONAL FORCE

The pull exerted by objects possessing mass on another mass is called gravitational forces. Gravitational force between two objects is a force of attraction. 

 

It is the gravitational force between the sun and the earth which holds the earth in its orbit around the sun. 

The force with which the earth is the objects towards it, is called the force of gravity or simply gravity. 

The force of gravity causes all the objects to fall towards the earth. 

The gravitational force of Earth or gravity acts on objects from a distance without there being a physical contact, therefore, gravitational force or gravity is an example of non contact forces. 

PRESSURE

 

Pressure is produced when a force acts on an object. The force acting on an object per unit surface area of the object is called pressure. 

The effect of a force depends on the area of the object on which it acts. 

Pressure = Force / Area

The SI unit of pressure is Newton per square metre ( N/m²), which is also called Pascal (Pa).

1 Pa = 1 N/m² 

1 kilo Pascal (1 kPa = 1000 Pa) is equal to 1000 Pascal. 

FACTORS ON WHICH PRESSURE DEPENDS

The pressure depends on two factors.

·         The force applied

·         Area over which force acts. 

The same force can produce different pressures depending on the area over which it acts. For example, when a force acts over a large area of an object, it produces small pressure. But if the same force acts over a small area of the object, it produces a large pressure. 

 

WHY SCHOOL BAGS HAVE WIDE STRAPS

A school bag or a shoulder bag has wide strap made of thick cloth so that the weight of bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder. And due to less pressure it is more comfortable to carry the heavy school bag. 

 

WHY A SHARP KNIFE CUTS BETTER THAN A BLUNT KNIFE

A sharp knife has a very thin edge to its blade. A sharp knife cuts objects like vegetables better because due to its very thin edge, the force of our hand falls over a very small area of the object producing a large pressure. And this large pressure cuts the object easily. On the other hand, a blunt knife has a thicker edge. A blunt knife does not cut an object easily because due to its thicker edge, the force of our hands falls over a larger area of the object and produces lesser pressure. This lesser pressure cuts the object with difficulty. 

 

WHY THE TIP OF A NEEDLE IS SHARP

The tip of a sewing needle is sharp so that due to its sharp tip, the needle may put the force on a very small area of the cloth producing a large pressure sufficient to Pierce the cloth being stitched. A knife Razer blade and an x are the cutting tools where is a sewing needle is a piercing tool. 

 

WHY THE DEPRESSION IS MUCH MORE WHEN A MAN STANDS ON THE CUSHION THEN WHEN HE LIES DOWN ON IT

When a man stands on a cushion then only his two feet having small area are in contact with the cushion. Due to this the weight of man falls on a small area of the cushion producing a large pressure. This large pressure causes a big depression in the cushion. On the other hand, when the same man is lying on the cushion then his whole body having larger area is in contact with the cushion producing much smaller pressure. This smaller pressure produces a very little depression in the cushion. 

 

PRESSURE EXERTED BY LIQUIDS

All the liquids exert pressure on the base and the walls of their containers. All the liquids have weight and this weight acts on the base of the vessel producing a pressure. 

The pressure exerted by a liquid increases with increasing depth inside the liquid. 

The pressure of a liquid increases with depth. The greater the depth of a point in a liquid the greater is the pressure. Liquids also exert pressure on the walls of the vessel in which they are stored. The sideways pressure exerted by liquids also increases with the depth of the liquid. 

The formation of mountains of water from the leaking pipes of water supply pipeline tells us that water exerts pressure on the walls of the container. 

A liquid exerts pressure in all directions even upwards. 

 

PRESSURE EXERTED BY GASES

 

All the gases exert pressure on the walls of their containers. Air pressure arises due to the constant solutions of the tiny molecules of the gases present in air with the walls of the container or vessel in which it is enclosed. 

High air pressure produces by the gas molecules on the walls of balloon causes it to extend and get inflated. 

 

Two examples that gases like air exert pressure are:

 

·         When air is filled into a balloon with our mouth the balloon gets inflated.

·         When air is filled into a bicycle tube with a pump the tube gets integrated and makes the tyre feel hard. 

 

ATMOSPHERIC PRESSURE

The layer of air above the earth is called atmosphere. Air has weight show the atmosphere consisting of tremendous amount of air has enormous weight the weight of atmosphere exerts a pressure on the surface of the earth and on all the objects on the Earth including ourselves. This pressure is known as atmospheric pressure. The atmospheric pressure is due to the weight of air present in the atmosphere above us. Atmospheric pressure also acts in all directions. 

 

MAGNITUDE OF ATMOSPHERIC PRESSURE

 

Atmospheric pressure is equal to the weight of air present in a BD tall column of air standing on one square metre area of the earth.  As we go up in the atmosphere from the surface of Earth the atmospheric pressure goes on decreasing. On the surface of Earth, the atmospheric pressure is maximum at the sea level. The atmospheric pressure on the top of a high mountain will be much less than at its base. 

Although pressure is is measured in Pascal, atmospheric pressure is measured in mm of Mercury. The atmospheric pressure on the surface of earth at the sea level is 760 mm of Mercury. 

 

OUR BODY AND ATMOSPHERIC PRESSURE

Change the atmospheric pressure acting on our body from outside is balanced by the blood pressure acting from inside we do not get crushed. But there is an effect of low atmospheric pressure in our body. At higher altitudes, atmospheric pressure becomes much less than our blood pressure. Since our blood is at a higher pressure than outside pressure, therefore, some of the blood vessels in our body burst and nose bleeding takes place at high altitudes. 

 

APPLICATIONS OF ATMOSPHERIC PRESSURE IN EVERYDAY LIFE

 

1. DRINKING STRAW

 

The drinking straw works on the existence of atmospheric pressure. The lower end of drinking straw is dipped in the soft drinks. When we suck at the upper end of the straw with our mouth, the pressure of air inside the straw and in our mouth is reduced. The pressure acting on the surface of the soft drink is equal to atmospheric pressure. So, the greater atmospheric pressure acting on the surface of the soft drink pushes the soft drink up the straw into our mouth. 

 

2. SYRINGE

 

A glass tube or plastic tube with a nozzle and piston for sucking in and ejecting liquid in a thin stream is called a syringe. The series works on the existence of atmospheric pressure. When the nozzle of a syringe is dipped in a liquid and its piston is withdrawn, the pressure inside the syringe is lowered. The greater atmospheric pressure acting on the surface of the liquid pushes the liquid up into the syringe. 

3. DROPPER

 

The dropper is a small glass tube with a rubber ball at one end and a nozzle at the other. Its functionality relies on atmospheric pressure. To use the dropper, we immerse the nozzle in the liquid and squeeze the rubber ball, causing air inside the tube and bulb to escape as bubbles. This action lowers the air pressure within the dropper. Once we release the rubber bulb, the higher atmospheric pressure outside the dropper forces the liquid to be drawn up into the tube.

 

4. RUBBER SUCKER

 

A rubber sucker, also known as a suction cup, is a rubber device that adheres firmly to flat and smooth surfaces when pressed. Its appearance resembles a small concave rubber cup. When we apply the rubber sucker to a flat, smooth surface and press it, the concave rubber cup flattens significantly, expelling most of the air from underneath. This compression results in a significant reduction in air pressure inside the rubber sucker. As a consequence, the higher atmospheric pressure from the outside effectively secures the rubber sucker in place on the flat surface.