Chapter 11 — Force And Pressure

What is a force? A force is a push or a pull that can change the state of rest or motion of an object or change its shape. Force is a vector (it has magnitude and direction). The SI unit of force is the newton (N), where 1 N = 1 kg·m/s².

Effects of a force

• It can start or stop motion, or change the speed or direction of motion (produce acceleration).
• It can change the shape or size of an object (deformation).
• It can produce rotation (torque or moment).

Types of forces

• Contact forces: forces that occur when bodies touch — e.g., friction, normal reaction, tension in a rope, applied push or pull.
• Non-contact (field) forces: forces that act at a distance — e.g., gravitational force, magnetic force, electrostatic force.

Balanced and unbalanced forces: When all forces on an object add up (vector sum) to zero, they are balanced and there is no change in motion. If the vector sum is non-zero, the forces are unbalanced and the object accelerates in the direction of the resultant force.

How we represent and measure force

• Graphically by arrows (length = magnitude, arrowhead = direction).
• Measured using instruments like a spring balance (in newtons) or a force sensor.

Key related concepts

• Weight: the gravitational force on a mass (W = mg).
• Normal reaction: the supporting force from a surface, equal and opposite to components of other forces perpendicular to that surface.
• Friction: a contact force opposing relative motion between surfaces.

Newton's laws (brief)

• First law (inertia): a body remains at rest or in uniform motion unless acted on by a net external force.
• Second law: the net force on a body is equal to mass × acceleration (F_net = ma).
• Third law: for every action there is an equal and opposite reaction.

Understanding forces uses simple vector addition (resultant force) and helps explain everyday phenomena — from pushing a book across a table to planets orbiting the Sun.

What is force? A force is a push or a pull that can change the state of motion of an object or deform it. Force is a vector (has magnitude and direction). The SI unit of force is the newton (N). A force of 1 N gives a mass of 1 kg an acceleration of 1 m/s².

Instruments used to measure force

• Spring balance (newton-meter): The most common classroom instrument. A spring balance has a spring fixed at one end and a hook at the other. When a load is hung, the spring stretches; the extension is shown on a calibrated scale which gives the force (weight) acting on the hook. It is usually graduated in newtons.
• Beam balance / weighing scale: Measures mass by comparison. A bathroom scale measures the normal reaction (which equals weight when at rest) and is calibrated to display mass in kg.

How a spring balance works

• Within its elastic limit, the extension of the spring is proportional to the applied force (Hooke's law). The scale is calibrated using known weights so the extension reading is given directly as force (N).
• To calibrate, hang known masses and mark the corresponding extensions. Convert the mass m (kg) to its weight W = mg (N) using g ≈ 9.8 m/s² and label the scale accordingly.

Measuring weight and its relation to mass

• Weight W is the gravitational force on a mass: W = mg. Here m is mass (kg) and g is acceleration due to gravity (~9.8 m/s² on Earth). Weight is measured in newtons.
• A spring balance measures weight (force). A beam balance compares masses and reads mass (kg) directly because it compares two masses under same g.

Precautions and limitations

• Do not exceed the elastic limit of the spring; otherwise Hooke's law fails and the balance will be inaccurate.
• Avoid jerks or quick motions; take steady readings. Read at eye level to avoid parallax error and ensure the pointer is at rest.
• Spring balances measure magnitude of force; direction must be noted separately.
• Calibration depends on local g; absolute conversion between mass and force uses local value of g.

Summary Measurement of force in Class 8 focuses on using instruments like the spring balance to measure forces (weights) in newtons, understanding W = mg, Hooke's law (F ∝ x for springs in elastic limit), calibration of scales, and being aware of sources of error.

What is friction? Friction is the force that opposes relative motion or tendency of motion between two surfaces in contact. It acts along the surfaces and always in a direction opposite to the motion or impending motion.

Why does friction occur? At the microscopic level, even seemingly smooth surfaces have irregularities (bumps and valleys). When surfaces contact, these irregularities and microscopic adhesion produce a resisting force — friction.

Types of friction

• Static friction: Acts when two surfaces are not moving relative to each other. It adjusts up to a maximum value (limiting friction) to prevent motion.
• Kinetic (sliding) friction: Acts when one surface slides over another. It is usually less than the maximum static friction and approximately constant for dry contact.
• Rolling friction: Resists the motion when an object rolls over a surface; usually much smaller than sliding friction.
• Fluid friction: Resistance experienced by objects moving through a fluid (liquid or gas). It depends on speed and fluid properties.

Key points / laws

• Static friction acts up to a maximum value: it can have any value from zero up to a limit depending on the applied force.
• Limiting static friction is generally greater than kinetic friction for the same surfaces.
• Friction depends on the nature of the surfaces and the normal (contact) force, not directly on contact area (for rigid bodies in simple cases).

Advantages and disadvantages

• Advantages: enables walking, driving, writing, holding objects, braking.
• Disadvantages: causes wear, heat, energy loss; makes motion harder in machines.

Ways to change friction

• To reduce friction: lubricants (oil/grease), polishing, using ball bearings, streamlining (for fluids).
• To increase friction: roughening surfaces, using rubber soles, adding treads on tires, increasing normal force.

Definition: Pressure is the force applied perpendicular to the surface of an object per unit area of that surface. Mathematically, pressure (P) = Force (F) / Area (A).

SI unit: pascal (Pa). 1 Pa = 1 N/m². Other common units: kPa, atm, mmHg, bar.

Understanding the idea: For the same force, a smaller contact area gives a larger pressure, and a larger contact area gives a smaller pressure. Thus pressure depends on two factors: the magnitude of the applied force and the area over which the force acts.

Pressure in solids: When a force acts on a solid surface (for example a nail pushed into wood), the stress concentrated over the nail’s small tip produces large pressure which allows penetration. If the same force is spread over a larger area (a blunt object), pressure is smaller and penetration does not occur.

Pressure in liquids and gases: Molecules in fluids (liquids and gases) exert pressure in all directions. In a liquid at rest, pressure increases with depth because of the weight of the liquid above. The pressure due to a liquid column of height h is given by p = ρgh (ρ = density, g = acceleration due to gravity). Total pressure at depth below the free surface equals atmospheric pressure plus ρgh.

Atmospheric pressure: The air around us exerts pressure on everything. We experience about 101325 Pa (≈1 atm) at sea level. Effects of atmospheric pressure can be demonstrated with a suction cup, a drinking straw, or a barometer.

Applications & principles: Pascal's principle: a change in pressure applied to a confined fluid is transmitted undiminished throughout the fluid. This is used in hydraulic lifts and brakes. Hydraulic machines use the relation F1/A1 = F2/A2 to multiply force.

Simple experiments to try:

• Press a nail and a blunt object with the same force onto soft clay to see difference in penetration.
• Stand on one heel or both feet to feel difference in pressure; try with shoes of different sole areas.
• Fill a plastic bottle with water, poke holes at different heights and observe flow rates to see increasing pressure with depth.

What is pressure in liquids?
Pressure in a liquid at a given point is the force exerted by the liquid per unit area on a surface in contact with it. It acts in all directions (downwards, sideways and upwards) at a point inside the liquid.

Key ideas

• Dependence on depth: Pressure in a liquid increases with depth. The deeper you go, the greater the pressure because the weight of the liquid column above the point increases.
• Independence from container shape and amount: For the same depth and same liquid, pressure does not depend on the shape of the container or the total volume of liquid. Two containers with different shapes but the same depth at a point have the same pressure at that depth.
• Pascal's principle: A change in pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid and to the walls of its container. This is the basis for hydraulic lifts and brakes.
• Atmospheric contribution: Measured pressure at a point inside a liquid is the sum of atmospheric pressure acting on the free surface plus the pressure due to the liquid column.

Why pressure acts in all directions
Because liquids have molecules that move and collide, a small element of liquid transmits forces to neighboring elements in every direction. Thus pressure at a point is scalar but results in forces on surfaces oriented any way.

Practical demonstrations

• Make holes at different depths in a bottle filled with water: water jets from deeper holes come out with greater force and reach farther — showing pressure increases with depth.
• Communicating vessels (two connected containers): liquid levels become the same on both sides regardless of container shapes — showing equal pressure at the same horizontal level.

Pascal's law (statement): When pressure is applied to a confined fluid, the pressure is transmitted undiminished in all directions throughout the fluid.

Conditions: The fluid must be confined and (for practical hydraulic systems) nearly incompressible and at rest.

Pressure and transmission: Pressure is defined as force per unit area: P = F / A. If a force F1 is applied on a small piston of area A1 in a connected fluid, the pressure produced is P = F1 / A1. According to Pascal's law, this same pressure acts on a larger piston of area A2, producing an output force F2 = P × A2 = (F1 / A1) × A2. Thus a small input force can produce a large output force if A2 > A1.

Mechanical advantage: The force multiplication (ideal, neglecting losses) is F2 / F1 = A2 / A1. Hydraulic systems trade force for distance: the larger piston moves a smaller distance than the smaller piston.

Why it works: Fluids transmit pressure equally; the rigidity of pistons and incompressibility of fluid ensure the applied pressure acts on all surfaces at the same level. Energy conservation implies force multiplication comes with reduced displacement on the larger piston.

Simple numeric example: If A1 = 2 cm² and A2 = 50 cm² and you push the small piston with F1 = 10 N, pressure P = 10 N / (2×10⁻⁴ m²) = 50,000 Pa. Output force F2 = P × A2 = 50,000 Pa × 5×10⁻³ m² = 250 N. So a 10 N input gives 250 N output (neglecting friction).

Common components in hydraulic systems: pistons/cylinders, fluid (oil), valves, pipes, and a pump for powered systems. Hydraulic systems are widely used where large forces and smooth control are required.

What is pressure?
Pressure is the force acting on a unit area of a surface. In symbols, P = F / A. In the SI system the unit of pressure is pascal (Pa), where 1 Pa = 1 N/m². Common larger units are kilopascal (kPa) and atmosphere (atm).

Pressure in gases — origin and behaviour
Gas pressure is caused by the continuous, rapid collisions of gas molecules with the walls of their container. Each collision exerts a tiny force on the wall; the sum of many collisions per unit area produces a measurable pressure.

• Dependence on number of molecules (n): More molecules (higher density) → more collisions per second → higher pressure (if volume and temperature are constant).
• Dependence on temperature (T): Increasing temperature increases the average speed of molecules → more frequent and more forceful collisions → pressure increases if volume is fixed.
• Dependence on volume (V): Reducing the volume (compressing the gas) increases collision frequency → pressure increases (if temperature is fixed).

These relations are summarized by simple gas laws (qualitatively for Class 8): at constant temperature, pressure is inversely related to volume; at constant volume, pressure increases with temperature.

Atmospheric pressure
Atmospheric pressure is the pressure exerted by the weight of the air column above a unit area of Earth’s surface. At sea level the atmospheric pressure is about 101,325 Pa (≈ 101.3 kPa) or 1 atmosphere (1 atm). It can be measured using a mercury barometer or an aneroid barometer.

How a mercury barometer works
A glass tube filled with mercury is inverted into a mercury reservoir. The column of mercury stabilizes at a height where the weight of the mercury column balances the atmospheric pressure. The relation used is P_atm = ρ g h, where ρ is the density of mercury, g is gravitational acceleration, and h is the height of the mercury column. At sea level h ≈ 760 mm (76 cm) of Hg.

Everyday effects of atmospheric pressure
Atmospheric pressure explains many common phenomena: why we breathe (air moves from high to low pressure), why liquids boil at lower temperatures at high altitudes (lower atmospheric pressure), why ears pop during altitude change (pressure difference across the eardrum), why suction cups stick (external atmospheric pressure pushes them onto surfaces), and why a sealed can can collapse if internal pressure is reduced (e.g., by condensing steam).

Measurements and safety notes
Pressure instruments include manometers (U-tube), mercury barometers, and aneroid barometers. When doing demonstrations with mercury, follow safety rules — many school demonstrations use water manometers or safe vacuum pumps instead.

What is pressure? Pressure is the force applied per unit area. It tells us how concentrated a force is. Mathematically, pressure = force/area. The SI unit is the pascal (Pa), where 1 Pa = 1 N/m².

Why area matters: For the same force, a smaller contact area produces a larger pressure and a larger area produces a smaller pressure. This explains why sharp objects cut easily and why snowshoes prevent sinking into snow.

Pressure in fluids: In liquids and gases, pressure acts in all directions. Hydrostatic pressure inside a fluid increases with depth and depends on the fluid density and gravity. Pascal's law states that a pressure applied to a confined fluid is transmitted equally in all directions — this is the basis of hydraulic machines.

Everyday significance: Many devices and natural phenomena use pressure principles: cutting tools, footwear design, vehicle tyres, pumps, hydraulic lifts, blood pressure in our bodies, barometers for weather, and water supply systems. Understanding pressure helps to design safer and more efficient tools and systems.

• Key qualitative points: smaller area → larger pressure; deeper in a liquid → larger pressure; confined fluid transmits pressure equally.
• Practical consequences: reduce pressure where you want to avoid damage (broad bases, cushions) and increase pressure where you need penetration or lift (knife tip, nail, hydraulic jack).