Grade 8 Chapter 3: Force and Pressure

Introduction

In our daily lives, we constantly interact with forces and experience pressure. From pushing a door open to feeling the weight of the air around us, these concepts are fundamental to understanding how the physical world works. This chapter delves into the different types of forces, their effects on objects, and the concept of pressure, including how it acts in solids, liquids, and gases, and its various applications.

Force

A force is defined as a push or a pull acting on an object. When a force is applied, it can bring about several changes in an object:

• Change the speed of a moving object (increase or decrease).
• Change the direction of motion of an object.
• Change the shape or size of an object.
• Bring a stationary object into motion.
• Stop a moving object.

Types of Force

Forces can be broadly classified into two main types based on whether physical contact is required between the interacting objects:

1. Contact Force

A contact force is a force that acts on an object by direct physical contact. Examples include:

• Muscular Force: The force applied by the action of muscles in living beings.
  • Examples: Lifting a heavy object, pushing a cart, kicking a football, carrying a bag.
• Mechanical Force: The force applied by means of a machine.
  • Examples: Force applied by a sewing machine, a bicycle, a tractor, or a car engine.
• Frictional Force: The force that opposes the motion of an object over a surface. It acts in the opposite direction of motion.
  • Examples: The force between the floor and your foot when walking, the force between the wheels of a vehicle and the road, braking a bicycle. Frictional force helps us walk without slipping.

2. Non-Contact Force

A non-contact force is a force that acts on an object without direct physical contact between the interacting objects. Examples include:

• Gravitational Force: The force of attraction exerted by the Earth on all objects towards its center. It is the force that pulls objects downwards.
  • Examples: A ball thrown upwards falls back down, rain falling, objects falling from a tree.
• Magnetic Force: The force exerted by a magnet on magnetic materials (like iron, nickel, cobalt) or on another magnet, either attracting or repelling.
  • Examples: A magnet attracting iron nails, two magnets repelling each other.
• Electrostatic Force: The force exerted by an electrically charged object on another charged or uncharged object.
  • Examples: A charged comb attracting small pieces of paper, static electricity when clothes stick together after drying.

Force and Motion

The state of motion of an object changes due to the application of a force. An object at rest remains at rest, and an object in motion continues to move with the same speed and in the same direction unless acted upon by an external, unbalanced force. This property is called Inertia.

• Balanced Forces: When two or more forces acting on an object are equal in magnitude and opposite in direction, they are called balanced forces. Balanced forces do not cause a change in the state of motion of an object.
• Unbalanced Forces: When the forces acting on an object are unequal in magnitude or not in opposite directions, they are called unbalanced forces. Unbalanced forces cause a change in the state of motion (speed or direction) of an object.

Pressure

We often observe that the effect of a force depends not only on its magnitude but also on the area over which it is applied. This concept is termed Pressure.

Pressure = Force / Area

The unit of force is Newton (N) and the unit of area is square meter (m²). Therefore, the SI unit of pressure is Newton per square meter (N/m²), which is also known as Pascal (Pa).

Factors affecting Pressure:

• Force: Pressure increases if the force increases (keeping area constant).
• Area: Pressure decreases if the area over which the force is applied increases (keeping force constant). Conversely, pressure increases if the area decreases.
• Examples:
  • A sharp knife cuts better than a blunt knife because the sharp edge has a smaller area, exerting more pressure for the same force.
  • A nail has a pointed tip so that it can be driven into a wall with less force.

Pressure on Solids

Solids exert pressure due to their weight. The pressure exerted by solids depends on the force (weight) and the area of contact.

• Bags with broad straps are easier to carry than bags with thin straps because the broad straps distribute the weight over a larger area, reducing the pressure on the shoulders.
• Tractors have broad tires to reduce pressure on the ground, allowing them to move easily over soft soil without sinking.

Pressure of Liquids (Fluid Pressure)

Liquids exert pressure not only downwards but also sideways and upwards on the walls and base of their container. This pressure increases with depth.

• Liquid exerts pressure on the walls of the container: If you take a plastic bottle and make holes at different levels, you will observe that water gushes out with greater force from the lower holes than from the upper ones. This demonstrates that liquid pressure increases with depth.
• Pressure increases with depth: The pressure exerted by a liquid column increases as the depth below the surface increases. This is why dams are built with thicker walls at the base.
• Pressure is exerted equally in all directions at the same depth: If you immerse a pressure measuring device in a liquid, you will find that the pressure at a certain depth is the same, regardless of the direction it faces.

Atmospheric Pressure

The air around us forms a layer called the atmosphere, which extends to a certain height above the Earth's surface. The weight of this column of air exerts pressure on us and everything on Earth. This is called Atmospheric Pressure.

• The atmospheric pressure at sea level is approximately 101 x 10³ Pascal (Pa) or 1 atmosphere.
• Atmospheric pressure decreases as altitude increases because the density of air decreases with height.
• Effects/Applications:
  • A rubber sucker sticks to a smooth surface due to atmospheric pressure pushing it.
  • We can drink with a straw because sucking air reduces pressure inside the straw, and atmospheric pressure on the liquid pushes it up.
  • A syringe works on the principle of atmospheric pressure to draw in liquid or inject it.
  • When we fill a water bottle and invert it, water does not fall if the mouth is covered by a piece of paper, due to atmospheric pressure supporting the paper.

Archimedes' Principle

When an object is wholly or partially immersed in a fluid (liquid or gas), it experiences an upward force called Buoyant Force. This buoyant force is equal to the weight of the fluid displaced by the immersed part of the object.

Principle: "When an object is partially or wholly immersed in a fluid, a force of buoyancy acts on it in the upward direction. This force is equal to the weight of the fluid displaced by the object."

• Buoyant Force: The upward force exerted by a fluid that opposes the weight of an immersed object.
• Whether an object floats or sinks depends on the relationship between its density and the density of the fluid.
  • If the object's density is less than the fluid's density, it floats.
  • If the object's density is greater than the fluid's density, it sinks.
  • If the object's density is equal to the fluid's density, it remains suspended.
• The buoyant force acting on an object depends on the volume of the liquid displaced by the object. A larger volume displaced means a greater buoyant force.

Applications of Archimedes' Principle:

• Ships and Submarines: Though very heavy, ships float because their hollow structure displaces a large volume of water, generating a buoyant force equal to their weight. Submarines use ballast tanks to control their buoyancy by taking in or expelling water.
• Hot Air Balloons: They float because the hot air inside is less dense than the cooler air outside, creating an upward buoyant force.
• Hydrometer: An instrument used to measure the density or relative density of liquids. It works on the principle that it sinks deeper in a less dense liquid and floats higher in a more dense liquid.

Density and Relative Density

• Density (ρ): Defined as mass per unit volume.

  Density = Mass / Volume

  The SI unit of density is kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³).
• Relative Density: It is the ratio of the density of a substance to the density of water at 4°C. Since it is a ratio of two similar quantities, it has no unit.

  Relative Density = Density of Substance / Density of Water

  (Density of water at 4°C is 1000 kg/m³ or 1 g/cm³)

Pascal's Principle

Pascal's principle, also known as Pascal's law, states that "pressure applied to an enclosed fluid is transmitted undiminished to every portion of the fluid and the walls of the containing vessel." This means that if you apply pressure to a fluid in a confined space, that pressure will be uniformly transmitted throughout the fluid.

Applications of Pascal's Principle (Hydraulic Systems):

This principle is the basis for hydraulic systems, which are used to multiply force. A small force applied to a small piston can generate a large force on a larger piston due to the uniform transmission of pressure.

• Hydraulic Jack: Used to lift heavy objects like cars. A small force applied to a small piston creates pressure, which is transmitted to a larger piston, generating a much larger lifting force.
• Hydraulic Brake: Used in vehicles to apply brakes. When the brake pedal is pressed, a small force generates pressure in the brake fluid, which is transmitted equally to the brake pads of all wheels, applying a large braking force.
• Hydraulic Press: Used for compressing materials or shaping metals.

Summary

• Force is a push or pull that can change an object's state of motion or shape.
• Forces are classified as contact (muscular, mechanical, frictional) or non-contact (gravitational, magnetic, electrostatic).
• Inertia is an object's resistance to change in its state of motion.
• Pressure is force per unit area (P = F/A), measured in Pascals (Pa). It decreases with increased area and increases with increased force.
• Liquids exert pressure in all directions, increasing with depth.
• Atmospheric pressure is the pressure exerted by the Earth's atmosphere, essential for various daily phenomena.
• Archimedes' Principle explains buoyant force, stating that the upward force on an immersed object equals the weight of the fluid it displaces. It explains why objects float or sink.
• Density is mass per unit volume (ρ = m/V). Relative density compares a substance's density to water's density.
• Pascal's Principle states that pressure applied to an enclosed fluid is transmitted equally throughout. It is the basis for hydraulic systems like jacks and brakes.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 3: Force and Pressure.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.