Class 8 Science Chapter 6 Force and Motion

Class 8 Science Chapter 6 Force and Motion is one of the most exciting and scoring chapters in your science syllabus. In this chapter, you will learn about force, rest and motion, speed, velocity, acceleration, levers, pressure, and atmospheric pressure. Whether you are solving numerical problems or preparing for theory questions, this complete guide to Class 8 Science Chapter 6 Force and Motion will make every concept simple and clear.

 Introduction to Force

• Force → A push or pull acting upon an object that changes (or tends to change) its state of rest or motion.
• Definition: Force is an external agency which changes or tries to change the state of rest or motion of a body.
• Unit: Newton (N).
• Examples:
  • Kicking a ball (force applied → ball moves).
  • Opening a door (push/pull).
  • Gravity pulling objects downward.

Exam Tip: Always mention “change in state of rest or motion” in the definition.

 Rest and Motion

• Rest: An object is said to be at rest if its position does not change with respect to its surroundings.
  • Examples: A book on a table, a parked car.
• Motion: An object is said to be in motion if its position changes with respect to its surroundings.
  • Examples: Running boy, flying bird.

Types of Motion:

1. Uniform Motion → Body covers equal distances in equal intervals of time.
   • Example: Car moving at 60 km/h throughout.
2. Non-uniform Motion → Body covers unequal distances in equal intervals of time.
   • Example: Car moving in traffic.

 Force and motion are relative terms:

• Rest and motion are not absolute. A passenger sitting in a moving bus is at rest relative to the bus but in motion relative to the ground.

Speed and Velocity

Speed

• Definition: Distance travelled per unit time.
• Formula:
  Speed = Distance ÷ Time   or    Distancetime
• Unit: m/s

Velocity

• Definition: Displacement per unit time in a particular direction.
• Formula:
  Velocity = Displacement ÷ Time or     Displacementtime
• Unit: m/s

Comparison between Speed and Velocity:

Feature	Speed (Scalar)	Velocity (Vector)
Type	Scalar	Vector (needs direction)
Direction	No direction	Has direction
Representation	Number only	Number + direction

Worked Example (Speed):

Q: A car travels 150 km in 3 hours. Find its speed.

• Step 1: Formula → Speed = Distance / Time.
• Step 2: Speed = 150 / 3 = 50 km/h.
• Answer: 50 km/h.

Worked Example (Velocity):

Q: A body moves 100 m east in 20 s. Find velocity.

• Step 1: Formula → Velocity = Displacement / Time.
• Step 2: Velocity = 100 / 20 = 5 m/s east.
• Answer: 5 m/s east

6.3.1 Relative Velocity

• Definition: The velocity of one object with respect to another.
• Formula in one dimension:
  • When two bodies move in the same direction:
    VAB = VA − VB
  • When two bodies move in opposite directions:
    VAB = VA + VB

Where:

• VAB = relative velocity of A with respect to B
• VA = velocity of A
• VB = velocity of B

Worked Examples:

1. Same direction: Car A = 60 km/h, Car B = 40 km/h.
   • Relative velocity = 60 – 40 = 20 km/h.
2. Opposite direction: Car A = 60 km/h east, Car B = 40 km/h west.
   • Relative velocity = 60 + 40 = 100 km/h.

6.3.2 Average Velocity

• Definition: Total displacement per total time taken.
• Formula:
• General formula:
  Average Velocity = Total Displacement ÷ Total Time
• If a body moves with two velocities u and v for equal time intervals:
  Average Velocity = (u + v) ÷ 2

• 6.3.3 Acceleration
• Definition: Rate of change of velocity with time.
• Formula:
  a = (v − u) ÷ t
• Where:
• u = initial velocity
• v = final velocity
• t = time
• Unit: m/s²
• Retardation: Negative acceleration (velocity decreases with time).

Worked Example:

Q: A car increases its velocity from 20 m/s to 40 m/s in 10 s. Find acceleration.

• Step 1: Formula → a=(v−u) / t 
• Step 2: a=(40−20)/10 m/s²
• Answer: 2 m/s².

6.4 Lever

• Fulcrum → Fixed point around which lever rotates.
• Load arm → Distance from fulcrum to load.
• Effort arm → Distance from fulcrum to effort.
• Load → Object lifted by lever.
• Effort → Force applied to lift the load.

Types of Levers:

1. First-class lever: Fulcrum between load and effort.
   • Examples: Scissors, crowbar, seesaw.
2. Second-class lever: Load between fulcrum and effort.
   • Examples: Wheelbarrow, nutcracker.
3. Third-class lever: Effort between fulcrum and load.
   • Examples: Human arm, fishing rod, broom.

Formula (Principle of lever):

Effort × Effort arm = Load × Load arm

 Mechanical Advantage (MA)

• Definition: Ratio of load to effort.
• Formula:

                     MA = LoadEffort

6.6 Velocity Ratio (VR)

• Definition: Ratio of distance moved by effort to distance moved by load.
• Formulas:

1. Using distances:

   VR = Effort distance / Load distance

   OR

   VR = Load distance / Effort distance

2. Using speeds:

   VR = Input speed / Output speed

   OR

   VR = Output speed / Input speed

6.7 Efficiency

• Definition: Ratio of useful work output to total work input × 100%.
• Formulas:
•   Using Work:
  η = (Output Work ÷ Input Work) × 100%
  OR
  η = (Input Work ÷ Output Work) × 100%
•   Using Mechanical Advantage (MA) and Velocity Ratio (VR):
  η = MAVR×100%
  OR
  η = VRMA×100%

Interesting Facts

• Newton’s first law → Object remains at rest/motion unless force acts.
• Levers were first described by Archimedes.
• Efficiency of a real machine is always < 100% due to friction.

Mnemonics / Memory Aids

• Types of levers: FLE → Fulcrum (1st), Load (2nd), Effort (3rd).
• Formulas sequence: S.V.A. → Speed, Velocity, Acceleration.

Summary / Quick Revision

• Force: Push or pull → changes state of rest/motion.
• Rest: No change in position; Motion: Change in position.
• Speed = distance/time; Velocity = displacement/time.
• Relative velocity depends on direction of motion.
• Acceleration = (v – u)/t. Retardation = negative acceleration.
• Levers classified as 1st, 2nd, 3rd class based on position of fulcrum, load, effort.
• MA = Load/Effort; VR = Effort distance/Load distance; Efficiency = (MA/VR) × 100%.

Pressure – Class 8 Science Chapter 6 Force and Motion

• Pressure (P): The normal force acting per unit area of surface.
• Formula:

    P=    FA

   Where:

• P = Pressure
• F = Force
• A = Area
• Units: Pascal (Pa) → 1 Pa = 1 N/m².

Examples:

• A knife cuts better with a sharp edge (less area → more pressure).
• A camel’s broad feet exert less pressure on sand (large area → less pressure).

Exam Tip: Always state unit of pressure as Pascal in SI.

6.8.2 Utilities of Pressure in Daily Life

1. Cutting tools (knife, axe) → sharp edge increases pressure, making cutting easier.
2. Syringe → pressure difference helps suck and push fluids.
3. Nails and pins → pointed ends exert high pressure to penetrate surfaces.
4. Camel walking on sand → broad feet reduce pressure, preventing sinking.

6.8.3 Liquid Pressure

• Definition: Pressure exerted by a liquid column due to its weight.
• Formula:

P = h × d × g

Where:

• h = height of liquid column
• d = density of liquid
• g = acceleration due to gravity

Relation:

• Pressure ∝ height of liquid column (greater depth → more pressure).
• Pressure ∝ density of liquid (denser liquids exert more pressure).

Examples:

• Deep-sea divers experience greater pressure.
• Dams have thick bases to withstand higher pressure at depth.

Utilities of Liquid Pressure in Daily Life (4):

1. Supply of water to households by water tanks (higher tank → more pressure).
2. Hydraulic presses and brakes (liquid pressure transmission).
3. Dams designed thicker at bottom due to greater pressure at depth.
4. Jet sprays (fire extinguisher, fountains).

Compressed Air Pressure

• Definition: Air stored under high pressure inside a container.
• Utilities (4):
  1. Inflating tyres, balls, balloons.
  2. Operating pneumatic tools (e.g., drills, hammers).
  3. Spray painting and perfumes.
  4. Breathing apparatus in scuba diving and aviation.

 Air Pressure Gauge

• Definition: Device used to measure compressed air or gas pressure.
• Applications: Used in tyres, gas cylinders, compressors.
• Units:
  • Pounds per square inch (psi).
  • Bar (1 bar ≈ 100,000 Pa).

Uses of Compressed Air (4):

1. Inflating vehicle tyres.
2. Powering pneumatic machines.
3. Cleaning machines in workshops.
4. Operating medical equipment (ventilators, inhalers).

Atmospheric Pressure

• Definition: Pressure exerted by air column on Earth’s surface due to weight of atmosphere.
• Reason: Air has mass → exerts force → creates pressure.
• Units:
  • Pascal (Pa).
  • Bar.
  • Millimeters of mercury (mmHg) or Torr (1 atm = 760 mmHg = 101325 Pa).

Examples:

• Sucking liquid through a straw → atmospheric pressure pushes liquid upward.
• Existence of vacuum-packed cans → external air pressure keeps them sealed.

 Measurement of Atmospheric Pressure

Manometer (U-tube type):

• Working Mechanism:
  • Consists of U-shaped glass tube partly filled with liquid (usually mercury or water).
  • One end open to atmosphere, other end connected to vessel.
  • Difference in liquid level indicates pressure difference.

Utilities of Atmospheric Pressure (4):

1. Drinking through straw, syringes.
2. Working of vacuum-sealed bottles/cans.
3. Functioning of barometer for weather forecasting.
4. Suction pumps and vacuum packaging.

Interesting Facts

• Highest atmospheric pressure ever recorded: 1083.8 mb in Siberia.
• At high altitudes, atmospheric pressure decreases → less oxygen → mountain sickness.
• Deep sea fish survive due to adaptation to extreme liquid pressure.

Mnemonics / Memory Aids

• Factors affecting liquid pressure: H-D-G → Height, Density, Gravity.
• Units of Pressure: P-B-M → Pascal, Bar, mmHg.

Summary / Quick Revision

• Pressure = Force/Area; SI unit = Pascal.
• Liquid pressure depends on height and density.
• Compressed air is used in tyres, tools, sprays, diving equipment.
• Air pressure gauge measures compressed air in psi or bar.
• Atmospheric pressure = 101325 Pa at sea level; measured using manometer/barometer.
• Atmospheric pressure enables straw drinking, suction pumps, weather prediction, packaging.

This completes the full revision of Class 8 Science Chapter 6 Force
and Motion