Force, Friction and Pressure - Complete Study Guide for Class 8 Physics

Force, Friction and Pressure

Force, friction, and pressure are fundamental concepts in physics that govern how objects interact with each other and their environment. From walking and driving to flying airplanes and drinking through a straw, these principles are at work in our daily lives.

Force is any influence that causes an object to undergo change in movement, direction, or shape. Friction is the resistive force between surfaces that opposes motion, enabling us to walk and drive. Pressure is force distributed over an area, essential for phenomena like atmospheric pressure and hydraulic systems.

What is Force?

A force is a push or pull that can produce at least one of the following effects:

1. Set a body at rest into motion: A football starts moving when kicked
2. Stop a moving body: A car stops after hitting a wall
3. Change the speed of a moving body: Acceleration or deceleration
4. Change the direction of motion: A cricket ball changes direction when hit by a bat
5. Change the shape or size of a body: Squeezing a sponge or stretching a rubber band

The direction of force is the direction in which the object is pushed or pulled.

Types of Forces

Forces are classified into two main categories based on whether physical contact is required:

1. Contact Forces

These forces act only when objects are in physical contact with each other.

(a) Muscular Force: Force exerted by muscles during physical activities. Examples include lifting objects, pushing doors, or holding items.

(b) Frictional Force: The force that opposes motion between surfaces in contact. It acts tangentially (parallel) to the contact surface.

(c) Normal Force: The perpendicular force exerted by a surface on an object in contact with it. It acts at right angles to the surface.

(d) Elastic Spring Force: Force exerted by compressed or stretched springs on objects in contact with them.

2. Non-Contact Forces

These forces act from a distance without physical contact.

(a) Magnetic Force: Force exerted by magnets on other magnets or magnetic materials like iron, cobalt, and nickel.

(b) Electrostatic Force: Force resulting from the attraction of opposite charges or repulsion of similar charges.

(c) Gravitational Force: The force by which objects attract each other due to their masses. Earth's gravity pulls all objects toward its center.

Units of Force

• SI Unit: Newton (N)
• CGS Unit: Dyne
• Practical Unit: Kilogram-force (kgf)
• Conversion: 1 kgf = 9.8 N (gravitational force on 1 kg mass)

Mass and Weight

Mass

Mass is the amount of matter in a body. It remains constant regardless of location.

Units:

• SI unit: Kilogram (kg)
• CGS unit: Gram (g)

Measurement: Mass is measured using a beam balance by comparing with known masses.

Weight

Weight is the gravitational force with which Earth pulls a body toward itself.

Formula: W = mg

Where:

• W = Weight (in Newtons)
• m = Mass (in kg)
• g = Acceleration due to gravity (9.8 m/s²)

Units: Same as force (Newton, kgf, gf)

Measurement: Weight is measured using a spring balance.

Difference: Mass is constant everywhere, but weight changes with the gravitational field strength.

Friction Force - The Necessary Evil

What is Friction?

Friction is the force acting along two surfaces in contact that opposes the motion or tendency of motion of one body over another.

Why Does Friction Occur?

Friction arises due to:

1. Adhesive forces between molecules of different materials at contact points
2. Surface irregularities that interlock when surfaces come together
3. Actual area of contact formed by microscopic contact points

When rough surfaces contact each other, atoms and molecules at these points attract through adhesive forces, creating resistance to motion.

Factors Affecting Friction

Friction depends on:

1. Nature of surfaces - Rough surfaces (grass) produce more friction than smooth surfaces (ice)
2. Normal force - Friction is directly proportional to the perpendicular force between surfaces
3. Actual area of contact - More contact area means more friction (independent of apparent area)

Note: Friction does NOT depend on:

• Apparent contact area
• Speed of motion (for most practical purposes)

Types of Friction

1. Static Friction

Friction when an object is not moving but has a tendency to move. It must be overcome to initiate motion.

2. Sliding (Kinetic) Friction

Friction when one object slides over another. Also called kinetic friction.

3. Rolling Friction

Friction when an object rolls over a surface. This is the smallest type of friction.

Relationship: Static friction > Sliding friction > Rolling friction

Limiting Friction

Limiting friction is the maximum frictional force that must be overcome to start motion. As applied force increases, friction also increases up to this limit. Once the applied force exceeds limiting friction, the object begins to move.

Friction in Fluids (Liquids and Gases)

When solids move through fluids, they experience frictional resistance called drag.

Relationship: f_solid > f_liquid > f_gas

Streamlined shape: A body shape designed to minimize fluid friction by allowing smooth flow around it. Examples include airplanes, submarines, rockets, and fish.

Effects of Friction

Negative Effects:

1. Opposes motion, requiring more energy
2. Produces heat, causing energy loss
3. Causes wear and tear of surfaces

Positive Effects:

1. Enables walking and running
2. Allows vehicles to move and brake
3. Enables writing with pens and pencils
4. Helps in gripping objects

How Friction Enables Walking

Walking is possible because of friction between our feet and the ground:

• Position 1: Friction acts forward (driving force)
• Position 2: No friction acts
• Position 3: Friction acts backward (decelerating force for stopping)

Ways to Reduce Friction

1. Streamlined shapes - Reduces fluid friction
2. Polishing - Makes surfaces smoother
3. Wheels - Converts sliding to rolling friction
4. Lubricants - Oil, grease create smooth layers between surfaces
5. Ball bearings - Use rolling friction instead of sliding

Thrust and Pressure

Thrust

Thrust is the total force acting perpendicular (normal) to a given surface area.

Unit: Newton (N)

Pressure

Pressure is the force acting perpendicularly per unit area.

Formula:

P = Thrust/Area = F/A

Units:

• SI unit: Pascal (Pa) or Newton per square meter (N/m²)
• 1 Pa = 1 N/m²
• Practical unit: Bar (1 bar = 10⁵ Pa)

For the same force, smaller area produces higher pressure. This is why a sharp needle penetrates easily while a blunt object doesn't.

Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the atmosphere at any location.

Standard Value:

• 1.01 × 10⁵ Pa at sea level
• Also called 1 atmosphere (1 atm)
• Equivalent to pressure by 760 mm mercury column

Variation: Atmospheric pressure decreases with altitude because air density decreases.

Measurement: Measured using a barometer.

Pressure in Liquids

Liquids exert pressure on container bases (due to weight) and walls (due to molecular collisions).

Important Properties of Liquid Pressure

1. Pressure is same at all points on the same horizontal level
2. Pressure increases with depth (deeper points experience more pressure)
3. Pressure depends on liquid density (denser liquids exert more pressure)
4. Liquids exert lateral pressure on container walls

Measurement: Liquid pressure is measured using a manometer a U-shaped tube with one arm open to air and the other connected to the measurement point.

Pascal's Law

Statement: In a fluid at rest, pressure applied at any point is transmitted equally in all directions throughout the fluid.

Formula: For a hydraulic system:

F₁/A₁ = F₂/A₂

Where F₁, F₂ are forces on pistons with areas A₁, A₂

Applications of Pascal's Law

1. Hydraulic brakes - Brake pedal force transmitted to wheels via brake fluid
2. Hydraulic press - Small force on small piston creates large force on large piston
3. Hydraulic jacks - Lift heavy vehicles
4. Hydraulic elevators - Move passengers between floors
5. Vacuum pumps and air compressors

The force multiplication factor equals the ratio of piston areas: F₂/F₁ = A₂/A₁

Buoyant Force (Upthrust)

What is Buoyant Force?

Buoyant force (or upthrust) is the upward force experienced by an object when immersed in a fluid, making it feel lighter.

Examples:

• Wood held underwater rises when released
• Stones feel lighter when lifted underwater
• Ships float on water

Cause of Buoyant Force

Buoyant force arises because:

1. Fluid pressure increases with depth
2. Bottom surface of immersed object experiences greater upward pressure than top surface experiences downward pressure
3. The net unbalanced force acts upward

Important Point: The deeper a point on the object, the greater the pressure at that point.

Factors Affecting Buoyant Force

Buoyant force depends on:

1. Volume of object immersed - Greater immersed volume produces larger buoyant force
2. Density of fluid - Denser fluids (like seawater) exert more buoyant force than less dense fluids (like fresh water)

Buoyant force does NOT depend on:

• Mass or density of the immersed object
• Depth of immersion (once fully submerged)
• Shape of the object

Archimedes Principle

Statement: When a body is fully or partially immersed in a fluid, the fluid exerts an upward buoyant force equal to the weight of the displaced fluid.

Formula:

Buoyant Force (B) = Weight of displaced fluid B = ρ_fluid × V_displaced × g

Where:

• ρ_fluid = Density of fluid
• V_displaced = Volume of fluid displaced
• g = Acceleration due to gravity

Application: This principle works for objects in both liquids and gases.

Principle of Floatation

When an object is released in a fluid:

1. If W > B (Weight > Buoyant force): Object sinks to the bottom
2. If B > W (Buoyant force > Weight): Object rises and floats with part above surface
3. If B = W (Equilibrium): Object floats in stable position

For a floating body in equilibrium:

Weight of liquid displaced = Weight of the body

This explains why:

• Ships made of steel (denser than water) float - their hollow design displaces enough water
• Ice floats on water - ice is less dense than liquid water
• Hot air balloons rise - hot air inside is less dense than surrounding cool air

Density and Relative Density

Density

Density is mass per unit volume of a substance.

Formula:

Density (ρ) = Mass/Volume

SI Unit: kg/m³

Density is a characteristic property - each substance has a specific density under given conditions.

Apparent Weight

Apparent weight is the weight of an object when immersed in a fluid.

Formula:

Apparent Weight = True Weight - Buoyant Force Apparent Weight = W - B

Objects feel lighter in water because buoyant force reduces the net downward force.

Relative Density (Specific Gravity)

Relative Density is the ratio of a substance's density to water's density.

Formula:

Relative Density = Density of substance/Density of water = Mass of substance/Mass of equal volume of water

Features:

• Has no units (dimensionless)
• Expresses how many times heavier/lighter a substance is compared to water
• Example: Iron has relative density 7.8, meaning iron is 7.8 times heavier than equal volume of water

Solved Examples

Example: An object has mass 10 kg. What is its weight on Earth? (g = 9.8 m/s²)

Solution:

Given: m = 10 kg, g = 9.8 m/s² Weight, W = m × g W = 10 × 9.8 = 98 N

Answer: Weight = 98 N

Example: A force of 20 N acts over an area of 4 cm². Find the pressure in N/m².

Solution:

Given: F = 20 N, A = 4 cm² = 4 × 10⁻⁴ m² Pressure = Force/Area P = 20/(4 × 10⁻⁴) = 5 × 10⁴ N/m²

Answer: Pressure = 50,000 Pa

Example: In a hydraulic lift, a force of 5 N is applied to a piston with area 1 m². If the other piston has area 5 m², find the force on the larger piston.

Solution:

Given: F₁ = 5 N, A₁ = 1 m², A₂ = 5 m² Using Pascal's law: F₁/A₁ = F₂/A₂ F₂ = (F₁ × A₂)/A₁ = (5 × 5)/1 = 25 N

Answer: Force on larger piston = 25 N

Example: A body of density 5 × 10³ kg/m³ and volume 4 m³ is immersed in a fluid of density 4 × 10³ kg/m³. Find the buoyant force. (g = 10 m/s²)

Solution:

Volume of body = Volume displaced = 4 m³ Mass of liquid displaced = Volume × Density = 4 × (4 × 10³) = 16 × 10³ kg Weight of liquid displaced = Mass × g = 16 × 10³ × 10 = 16 × 10⁴ N = 160,000 N Buoyant Force = 1.6 × 10⁵ N

Answer: Buoyant force = 160,000 N

Example: Convert 5 kgf to Newtons.

Solution:

1 kgf = 9.8 N 5 kgf = 5 × 9.8 = 49 N

Answer: 5 kgf = 49 N

Example: Calculate the pressure at a depth of 10 m in water. (Density of water = 1000 kg/m³, g = 10 m/s²)

Solution:

Pressure at depth = ρgh P = 1000 × 10 × 10 = 100,000 Pa = 10⁵ Pa

Answer: Pressure = 100 kPa or 1 bar

Example: A stone weighs 100 N in air. When immersed in water, it experiences a buoyant force of 30 N. What is its apparent weight?

Solution:

Apparent Weight = True Weight - Buoyant Force Apparent Weight = 100 - 30 = 70 N

Answer: Apparent weight = 70 N

Example: An object has mass 500 g and occupies 250 cm³. Find its relative density.

Solution:

Volume of object = 250 cm³ Equal volume of water has mass = 250 g (density of water = 1 g/cm³) Relative Density = Mass of object/Mass of equal volume of water = 500/250 = 2

Answer: Relative Density = 2 (object is 2 times denser than water)

Example: A block requires a minimum force of 50 N to start moving on a surface. What is the limiting friction?

Solution:

Limiting friction = Minimum force needed to start motion Limiting friction = 50 N

Answer: Limiting friction = 50 N

Example: Two identical forces act on surfaces of area 2 m² and 4 m². Compare the pressures.

Solution:

Let force = F Pressure₁ = F/2 Pressure₂ = F/4 Ratio = P₁/P₂ = (F/2)/(F/4) = 4/2 = 2

Answer: Pressure on smaller area is 2 times greater

Example: A wooden block of mass 500 g floats on water. How much water does it displace?

Solution:

For floating equilibrium: Weight of block = Weight of water displaced Mass of water displaced = Mass of block = 500 g Volume of water displaced = 500 cm³ (since density of water = 1 g/cm³)

Answer: 500 cm³ or 0.5 L of water displaced

Example: A hydraulic press has pistons of area 10 cm² and 100 cm². What is the mechanical advantage?

Solution:

Mechanical Advantage = A₂/A₁ = 100/10 = 10 This means a force of 1 N on small piston produces 10 N on large piston

Answer: Mechanical Advantage = 10

Example: An astronaut has mass 60 kg. Her weight on Earth is 588 N. What would be her mass on the Moon?

Solution:

Mass is independent of location and remains constant Mass on Moon = Mass on Earth = 60 kg (Note: Her weight would be different due to Moon's weaker gravity)

Answer: Mass remains 60 kg

Example: A substance has mass 2400 g and occupies a volume of 300 cm³. Find its density in SI units.

Solution:

Mass = 2400 g = 2.4 kg Volume = 300 cm³ = 300 × 10⁻⁶ m³ = 3 × 10⁻⁴ m³ Density = Mass/Volume = 2.4/(3 × 10⁻⁴) = 0.8 × 10⁴ = 8000 kg/m³

Answer: Density = 8000 kg/m³

Example: An object has density 800 kg/m³. Will it float or sink in water (density 1000 kg/m³)?

Solution:

Object density (800 kg/m³) < Water density (1000 kg/m³) When density of object < density of liquid, it floats

Answer: The object will float

Example: Two identical objects are immersed at the same depth in water (density 1000 kg/m³) and oil (density 800 kg/m³). Which experiences greater pressure?

Solution:

Pressure = ρgh For same depth h: P_water = 1000 × g × h P_oil = 800 × g × h P_water > P_oil

Answer: Object in water experiences greater pressure

Example: A stone has weight 200 N in air. When completely submerged in water, buoyant force is 60 N. What force is needed to lift it underwater?

Solution:

Net downward force underwater = Weight - Buoyant force = 200 - 60 = 140 N Force required to lift = 140 N (upward)

Answer: 140 N force needed

Example: Atmospheric pressure is 1.01 × 10⁵ Pa. What force does air exert on a window of area 2 m²?

Solution:

Force = Pressure × Area F = 1.01 × 10⁵ × 2 F = 2.02 × 10⁵ N = 202,000 N

Answer: Force = 202 kN (balanced by equal force from inside)

Example: Ice has relative density 0.9. What percentage of an iceberg is submerged in seawater?

Solution:

For floating: Weight = Buoyant force ρ_ice × V × g = ρ_water × V_submerged × g V_submerged/V = ρ_ice/ρ_water = 0.9 Percentage submerged = 0.9 × 100 = 90%

Answer: 90% submerged, 10% above water

Example: A 5 kg block on a horizontal surface experiences a forward push of 60 N and friction of 35 N. What is the net force?

Solution:

Forward force = 60 N Backward friction = 35 N Net force = 60 - 35 = 25 N (forward) This will cause acceleration = F/m = 25/5 = 5 m/s²

Answer: Net force = 25 N forward

Formulas Chart

Practical Applications

In Daily Life

• Walking and running - Friction between shoes and ground
• Writing - Friction between pen and paper
• Drinking through straw - Atmospheric pressure difference
• Suction cups - Atmospheric pressure
• Sharp knives - Pressure concentration on small area

In Technology

• Hydraulic systems - Brakes, lifts, presses (Pascal's law)
• Ships and submarines - Buoyancy (Archimedes principle)
• Airplanes - Streamlining to reduce air resistance
• Ball bearings - Reducing friction in machinery
• Dams - Designed considering water pressure at depth

Tips for Students

1. Understand concepts, don't memorize - Focus on why phenomena occur, not just what happens
2. Practice numerical problems - Work through diverse examples to build confidence
3. Relate to real-life - Connect physics concepts to everyday experiences
4. Draw diagrams - Visualize forces, pressure distributions, and buoyancy
5. Compare and contrast - Understand differences between related concepts (mass vs weight, static vs kinetic friction)
6. Use correct units - Always write units in calculations and check dimensional consistency
7. Review formulas regularly - Understand when and how to apply each formula

Conclusion

Force, friction, and pressure are interconnected fundamental concepts that explain countless phenomena in our physical world. Understanding these principles enables us to:

• Explain why objects move or remain stationary
• Design efficient machines and transportation systems
• Understand natural phenomena like weather patterns and ocean currents
• Solve practical engineering problems
• Appreciate the physics behind everyday experiences

Learn these concepts provides a strong foundation for advanced physics topics and develops problem-solving skills applicable across scientific disciplines.

This comprehensive study guide has been prepared by experienced physics educators with expertise in Class 8 curriculum. The content follows NCERT guidelines and incorporates best practices in science education to ensure conceptual clarity and practical relevance.