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Force And Pressure Class 8 Notes - Chapter 11




Class 8 Science Notes Chapter 11 Force and Pressure

Meaning of force

  • Force can be defined as a push or pull on an object. Both non-loving and living things can exert a force.
  • A force can be defined as a push or pull upon an object resulting from the object interaction with another object.
  • For example-Picking, Kicking, Shutting, Squeezing, Pushing, Lifting etc.
  • Force can also be defined as an influence that brings about a change in the speed, direction of motion, or shape of the object.
  • In order to move an object from one location to another, there must be a net force exerted on the object to trigger the object in motion.
  • More push is required to move a heavy object in comparison to a lighter object.
push and pull force

Question Can you lift a bucket of water without holding it?

Answer A pull action is required to lift a bucket. This is provided by the force exerted by the body holding the bucket. As a result, we need a holding to lift a bucket. This force is produced by the action of our body's muscles.

Question: Give two examples of each of the situations in which you push or pull to change the state of motion of objects?

Answer
Examples of pushing:
(i) Player pushing a football using his foot.
(ii) Pushing an open drawer to close it.

Examples of pulling:
(i) pulling a toy car with thread to move
(ii) Opening a drawer of a table.

What causes the force to arise?

  • The interaction between two objects arises from the force.
  • Force includes gravity, friction and force applied on an object in order to move it. The force causes a change in the speed or direction of motion.
  • An object will either be in motion or at rest as a result of forces. If an object is at rest and remains at rest, the forces acting on it are said to be balanced. For it to begin moving, we must apply additional force.
  • When force is applied, two objects are involved: one that applies the force and one that receives it.
  • For example- A football placed on the ground is at rest position. For it to move in any direction, some force should be applied to the ball. In order to move the ball player can hit the ball.

Force arises due to interaction

  • A force arises due to interaction between objects. When applying a force to an object, the interaction between object and the source of force is required.
  • For example- We can exert a force on a ball to move in any directions. It happens when we kick the ball, so the force that arises is due to the interaction between feet of the player and the ball.

Effects of force

Let us now look at some of the characteristics of force

  1. A force can change the size and shape of an object. For example – While playing with clay we can change the shape of clay by applying force with our hands. The shape of rubber band changes when it is stretched.
  2. A force can change the speed of a moving object. Force can slow down or completely stop a moving object. To do this we need to apply a force in a direction opposite to the direction of motion of the moving body.
  3. A force can move an object which is at rest. For example – A stationary trolley or trolley which is at rest can be made to move by applying some force. So the motion of object can be halted by suitable application of force.
  4. A force can change the direction of motion of a moving object. For example – A football can changes its direction by kicking it in a different direction.

Note – Change in the mass of an object is not an effect of force.

Question: A blacksmith hammers a hot piece of iron while making a tool, How does the force due to hammering affect the piece of iron?

Answer A blacksmith exerts muscular force when hammering a hot piece of iron. This muscular force modifies the shape of the iron, enabling it to be shaped as needed.

Types Of Forces

There are two types of forces namely contact force and non-contact force

1. Contact forces

These are the kinds of force that came into action when two interacting bodies are in physical contact with each other. Muscular force and frictional force is an example of contact force.

Types of Contact Force

a. Muscular force
  • It is a type of contact force. It is the force that is exerted by the muscles of the body.
  • For example- To lift an object we use a hand. If we want to pull a bucket of water or lift a heavy box it requires muscular force.
  • Like humans, animals also use muscular force.
b. Frictional force -
  • The force that causes objects to slow down and stop moving is called frictional force. Friction always acts in the direction opposite to the direction of motion of the object.
  • For example-This is due to the force of friction when a ball comes to rest after some time.
  • This happens due to friction between the ground and the ball.

2. Non-contact forces

These are the kinds of force that is came into action when two interacting bodies are not in physical contact with each other.

Types of Non-Contact Force

a. Magnetic force -
  • Magnetic force is the force exerted by a magnet on another magnet or on a piece of iron.
  • Because a magnet is surrounded by an unseen magnetic force field, it can act from a distance.
  • For example- To separate the iron objects from the garbage since a magnet can attract a piece of iron as the force between them is an attractive type of force.
    A magnet being used to pick scraps of iron
b. Electrostatic force –
  • The force exerted by a charged body on another charged or uncharged body.
  • So, the force exerted by an electrostatic charge is referred to as an Electrostatic Force.
  • For example-Lightning. This force occurs due to electric charges. It can be both attractive and repulsive in nature.
  • It is because of electrostatic force that bits of paper move towards the comb which was rubbed through dry hairs. This happens because when comb is rubbed through your dry hairs it acquires a property called electric charge.
  • Figure below shows Electrostatic force acting on bits of paper from a distance
    Electrostatic force acting from a distance
c. Gravitational force –
  • The force that pulls the objects towards the earth.
  • It is the force of attraction between the particles of matter.
  • For example – The falling of an apple from a tree.
    The falling of an apple from a tree

Question: Name the forces acting on a plastic bucket containing water held above ground level in our hand.

Answer: Muscular force and gravitational force are the forces acting on the plastic bucket.


Magnitude Of Forces

  • By term magnitude of forces, we mean the net amount of all the forces being applied to the object. It is the sum of all forces operating on a body.
  • When two forces act in the same direction, the magnitude of the force increases. It is equal to the sum of both forces.
  • When two forces act in opposite directions, the magnitude of the force becomes less or decreases. It is the difference between the two forces.

How can we express the force?

A force is expressed by its magnitude and direction. The magnitude of the force is a numerical value. The direction of force is away from us when we push something and towards us when we pull something.

Units of force

The (Standard international system of the unit ) SI unit of force is called Newton.

The unit of force is named after the renowned scientist Sir Issac Newton.

The unit of force can be defined as the force needed to accelerate a body weighing 1kg by 1 meter per second.

F = m × a

Force Concept Map

force concept map

Pressure

Pressure is defined as the force acting on a unit area of a surface. The effect of force is measured by this quantity.

It can be represented as

$Pressure = \frac{Force}{Area}$

Or,

Pressure is directly proportional to force and inversly proportional to the area over which it is applied.

So, if we decrease the amount of area, pressure increases and if we increase the amount of area, pressure decreases.

Unit of pressure

The SI unit of pressure is Pascal.

$1Pa = 1 N/m^2$

Applications of pressure in our daily life

  • Broad Straps of shoulder bags:- The straps of shoulder bags are made broader which increases the area of contact and reduces the pressure on the shoulders. We know that pressure is inversely proportional to area, thus the wider the area, the less pressure is exerted, making it easier to carry a shoulder bag with a wide strap.
  • Elephants have broader feet, which increases the area of contact with the ground and results in lower pressure and enables the elephants to walk easily.
  • It is easier to hammer a sharp iron nail than to hammer a blunt one because the area of a sharp iron nail is smaller than that area of a blunt nail. So, it becomes easier to hammer the nail into the wall.
  • The use of skies helps to walk on snow as skies increase the area of contact between our foot and snow. This reduces pressure on the snow.
  • Peeling vegetables with a blunt knife is tough. It is much easier to peel vegetables using a sharp knife that has sharp edges rather than a blunt knife. This is because the sharp edge of knife has a small surface area and less force delivers the amount of pressure needed to cut or peel vegetables.

Pressure Exerted by liquids and Gases

  • It is the force exerted by liquids or gases per unit area.
  • Liquids and gases exert pressure in all directions. The pressure inside a them increase with depth.

Pressure Exerted By Liquids

  • Liquids exert pressure due to the weight that liquid possesses.
  • The liquid posses pressure in the downward direction.
  • The pressure exerted by liquids at the bottom depends on the height of the liquid column. The pressure exerted by liquids increases with depth.
  • For example – The body of the submarine is thick and built by strong material because the pressure of water is high in deep-sea so to withstand that pressure submarine is thick.
  • Liquids also exert pressure in all directions on the walls of the container.
  • For example – Take a container and make a hole in it. Now close the holes with tape and fill the container with water. Then remove the tape. We will observe that water comes out of the container in all direction.
  • This shows that liquids exert pressure in all direction.

Pressure Exerted by Gases

  • Gases too exert pressure on the walls of container like liquids. Air pressure arises due to constant collisions of tiny molecules of gases present in the air with the walls of the container it is enclosed.
  • For example- When air is pumped into a bicycle tube by using a pump, the bicycle gets inflated due to air pressure exerted by the collisions of gas molecules in air with the inner walls of the rubber tube. For this reason, bicycle tyres feel hard.

Atmospheric Pressure

  • Atmosphere – The blanket of air that surrounds the earth is called the atmosphere.
  • All fluids (liquids and gases) exert pressure. So the air also exerts a pressure called atmospheric pressure.
  • Atmospheric pressure is the pressure within the atmosphere of Earth. Atmospheric pressure is due to the weight of air present in the atmosphere above us. This pressure causes air molecules to be more tightly packed together than those that are high in the atmosphere. On the surface of the earth, the atmospheric pressure is maximum at sea level.
  • Atmospheric pressure also exists in all directions. The instrument used for the measurement of atmospheric pressure is known as a barometer. There are two types of barometer-
    1. Mercury barometer
    2. Aneroid barometer
  • The mercury barometer is an ideal one as it gives the standard unit of atmospheric pressure.
  • In a mercury barometer, a column of mercury in a glass tube rises or falls the weight of atmospheric pressure changes.
  • The approximate value of atmospheric pressure on the earth’s surface is $1.013 \times 10^5 pascal$.
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