📘Class 9 Science – Chapter: Work and Energy Notes

✨ Introduction to Work and Energy

• Work and energy are fundamental concepts in physics related to force and motion.

• Work is said to be done when a force is applied on an object and the object moves in the direction of the force.

Example: Lifting a bucket, pushing a cart.

🔹 Work

• Work = Force × Displacement × cosθ

Equation (simple font):
Work (W) = F × d × cosθ

Where:

• F = Applied force

• d = Displacement

• θ = Angle between force and displacement direction

Conditions for Work:

• Force must be applied.

• Displacement must occur.

• Component of force must be in direction of displacement.

Example: Pushing a wall with no movement = zero work.

🔹 Units of Work

• SI unit: Joule (J)

• 1 Joule = Work done when 1 N force moves an object by 1 meter

🔹 Energy

• Capacity to do work

• Scalar quantity

SI Unit: Joule (J)

Types of Energy:

• Kinetic Energy

• Potential Energy

• Mechanical Energy

🔹 Kinetic Energy (KE)

• Energy possessed by a body due to its motion.

Formula:
KE = 1/2 × m × v²

Where:

• m = mass

• v = velocity

Example: A moving car has kinetic energy.

🔹 Potential Energy (PE)

• Energy possessed by a body due to its position or configuration.

Formula:
PE = m × g × h

Where:

• m = mass

• g = gravity (9.8 m/s²)

• h = height from ground

Example: Water stored in a tank at height.

🔹 Mechanical Energy

• Sum of kinetic and potential energy.

Formula:
Total Mechanical Energy = KE + PE

🔹 Law of Conservation of Energy

• Energy can neither be created nor destroyed, only transformed.

• Total energy in a closed system remains constant.

Example: Falling ball – PE converts into KE.

🔹 Power

• Rate of doing work.

Formula:
Power = Work / Time

SI Unit: Watt (W)

1 Watt = 1 Joule / 1 second

Commercial Unit of Energy:

• kilowatt-hour (kWh)

• 1 kWh = 3.6 × 10⁶ J

Example: A machine does 1000 J work in 2 s. Power = 1000 / 2 = 500 W

🔹 Work-Energy Theorem

• Work done by the net force on an object = Change in its kinetic energy

Equation:
W = ΔKE = 1/2 × m × (v² – u²)

Where:

• u = initial velocity

• v = final velocity

🔹 Comparing Energy Consumption