Work and Energy Class 9 Science — Revision Notes
This chapter covers work, energy, and power. Students learn about kinetic and potential energy, conservation of energy, and practical applications.
TL;DR: This chapter covers work, energy, and power. Students learn about kinetic and potential energy, conservation of energy, and practical applications.
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
This chapter covers work, energy, and power. Students learn about kinetic and potential energy, conservation of energy, and practical applications.
Work
- Work = Force x Displacement x cos(θ) where θ is angle between force and displacement
- Work = Fs (when force is along direction of motion)
- Unit of work is joule (J): 1 J = 1 N x 1 m
- Work is zero when force perpendicular to displacement
Energy Types
- Kinetic energy: energy due to motion (KE = 1/2 mv^2)
- Potential energy: energy due to position (PE = mgh)
- Mechanical energy: sum of kinetic and potential energy
- Other forms: thermal, electrical, chemical, nuclear
Work-Energy Theorem
- Work done equals change in kinetic energy: W = KE_final - KE_initial
- W = 1/2 m (v_f^2 - v_i^2)
- Net work causes change in kinetic energy
- Allows calculation of final velocity from work done
Power
- Power = Work / Time = Energy / Time
- Unit of power is watt (W): 1 W = 1 J / 1 s
- 1 horsepower = 746 watts
- Power indicates rate at which work is done
Key Terms
- Work: Product of force and displacement in direction of force (F x s x cos θ)
- Energy: Capacity to do work
- Power: Rate at which work is done (P = W/t)
Frequently Asked Questions
Is work done when carrying book horizontally?
No, work is zero. Weight acts downward, displacement is horizontal. Angle θ = 90°, cos 90° = 0, so W = F x s x 0 = 0.
What is conservation of mechanical energy?
In absence of friction, total mechanical energy (KE + PE) remains constant. KE + PE = constant. As object falls, PE decreases and KE increases equally.
More Class 9 Science Revision Notes
- Matter in Our Surroundings
- Is Matter Around Us Pure
- Atoms and Molecules
- Structure of the Atom
- The Fundamental Unit of Life
- Tissues
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