Quick Revision Notes: Class 8 Science

Chapter: Force and Pressure

MP Board Education

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1. Introduction to Force (बल)

• Force: A push or a pull on an object is called a force. Force can bring objects into motion, stop them, change their speed, or change their direction.
• Unit of Force: Newton (N)
• Effects of Force:
  • Can change the state of motion of an object (from rest to motion, or motion to rest).
  • Can change the speed of an object.
  • Can change the direction of motion.
  • Can change the shape (deformation) of an object.

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2. Types of Forces (बल के प्रकार)

Forces are broadly classified into two main categories based on their application:

A. Contact Forces (संपर्क बल)

Forces that act only when objects are in physical contact with each other.

1. Muscular Force (पेशीय बल): Force resulting from the action of muscles (e.g., lifting a school bag, running).
2. Frictional Force (घर्षण बल): The force that opposes the motion of an object over a surface (e.g., a rolling ball stopping on its own).

B. Non-Contact Forces (असंपर्क बल)

Forces that act from a distance without any direct physical contact between the objects.

1. Magnetic Force (चुंबकीय बल): Force exerted by a magnet on other magnets or magnetic materials like iron.
2. Electrostatic Force (स्थिरवैद्युत बल): Force exerted by a charged body on another charged or uncharged body (e.g., bits of paper attracted to a rubbed plastic comb).
3. Gravitational Force (गुरुत्वाकर्षण बल): The attractive force exerted by the Earth (or any massive body) on all objects towards its center.

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3. Pressure (दाब)

• Definition: The force acting on a unit area of a surface is called pressure.
• Formula: $$\text{Pressure (P)} = \frac{\text{Force (F)}}{\text{Area (A)}}$$
• SI Unit of Pressure: Pascal ($\text{Pa}$) or $\text{N/m}^2$ (Newton per square meter).
• Key Relations:
  • Pressure is directly proportional to Force ($P \propto F$). If force increases, pressure increases.
  • Pressure is inversely proportional to Area ($P \propto \frac{1}{A}$). Smaller the area, greater is the pressure for the same force.

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4. Pressure Exerted by Liquids and Gases (तरल तथा गैसों द्वारा डाला गया दाब)

• Liquid Pressure:
  • Liquids exert pressure on the walls of the container.
  • Liquid pressure increases with the depth of the liquid.
  • Liquids exert equal pressure at the same depth in all directions.
• Atmospheric Pressure (वायुमंडलीय दाब):
  • The envelope of air surrounding the earth is called the atmosphere, and the pressure exerted by this air is known as atmospheric pressure.
  • Atmospheric pressure decreases as we go higher up into the mountains.

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5. Important Terms & Summary for Quick Recall

• Resultant Force: When two or more forces act on an object in the same direction, they add up. If they act in opposite directions, the net force is the difference between them.
• Balanced Force: When the net force acting on an object is zero, it does not change the state of rest or motion.
• Unbalanced Force: When the net force is not zero, it causes the object to move or change its speed/direction.
• Why do sharp knives cut better than blunt ones? Because a sharp knife has a very small surface area, which exerts high pressure on the object with a small force.