CBSE Notes Class 8 Science Chapter 8 Force and Pressure

In physics, force and pressure are foundational concepts that elucidate the interactions between objects and their environments. Grasping these ideas is essential for understanding a range of phenomena, from everyday occurrences to complex scientific principles. While force and pressure are interconnected, they represent distinct concepts that shape our comprehension of and interactions with the physical world.

1.0Force

• Force is an action applied to an object that can alter its state of rest or change its motion.
• A push or pull acting on some objects is called a Force.SI Unit of Force is Newton and CGS Unit is Dyne.
• 1N=105Dyne

2.0Effect of Forces

• When two objects interact with each other, a force is exerted between them.
• Forces are always exerted by one object on another object i.e to apply force at least two objects are required.
• Effect of force depends on both magnitude and direction. Force is a vector quantity. A force vector indicates the direction of the force and its length is proportional to the force's magnitude.
• Force can bring a change in the state of motion.
• A force can either accelerate or decelerate a moving object.
• A force can alter the direction of a moving object.
• A force can induce rotational movement in an object.
• Force can change the shape of an object.

3.0Types of Forces

1. Contact Forces-A force that is exerted only when two objects are touching is called contact forces. Some examples of Contact forces are:

• Muscular Force-The force resulting due to the action of muscles.
• Friction-Force which is responsible for changing the state of motion of objects. Frictional force arises from the contact between surfaces.
• Tension- It is a force exerted by string,ropes when they are pulled.When an object pulls on a string or cable, it creates tension, which is the force transmitted through the medium. This force acts along the length of the medium and is directed away from the object creating the pull.

2. Non-Contact Forces-A non -contact force is a force that one object exerts on another when they are not touching. Some examples of non contact forces are:

• Magnetic Force-The force exerted by a magnet on a piece of iron or on another magnet. Similar poles repel each other, whereas opposite poles attract.
• Electrostatic Force-The force exerted by a charged body on another charged body or uncharged body is known as electrostatic force. The electrostatic force between two charges can be attractive or repulsive.
• Gravitational Force-Gravitational force is the attractive force that exists between any two objects with mass.Objects fall towards earth because it pulls them this force is called gravitational force or force of gravity.

4.0Pressure

• Pressure is the force per unit area. It measures how force is distributed over a surface. 

$\text{pressure}=\frac{Force}{Area}$

• SI Unit of Pressure is Pascal.
• Pressure is inversely proportional to the area of contact, so the smaller the area, the larger is the pressure on a surface for the same force.

5.0Pressure Exerted by Liquid and Gases

• Pressure in a liquid is the force exerted per unit area on the walls of its container or on submerged objects. This pressure results from the weight of the liquid above the point where the measurement is taken.
• In liquids, pressure increases with depth and is applied uniformly in all directions. This pressure is influenced by the liquid's density, gravitational acceleration, and the depth below the surface.
• In gases, pressure is influenced by both the volume and temperature. Changes in either the volume or temperature can cause variations in the gas pressure.

6.0Atmospheric Pressure

• Atmospheric pressure is the force exerted by the Earth's atmosphere on each unit area of the Earth's surface. This pressure arises from the weight of the air above and changes with altitude, weather conditions, and other factors.