Centre of Gravity and Centre of Mass – Physics Class 9

Centre of Gravity and Centre of Mass — Class 9 Physics Lecture Summary

Lecture Overview

This conceptual lecture explores the Centre of Gravity (CG) and Centre of Mass (CM). The teacher explains how complex, large bodies can be simplified into a single point for physics calculations.

Main Concepts Explained by the Teacher

The teacher defines the Centre of Mass as the point where an applied force causes the body to move in a straight line without spinning. The Centre of Gravity is defined as the exact point where the total weight of the body appears to act vertically downward. For symmetric, regular objects (like a uniform ruler or a sphere), the CG is exactly in the geometric center.

Important Definitions and Terms

• Centre of Mass: A point where an applied force causes only linear motion without rotation.
• Centre of Gravity: A point through which the entire weight of the body acts downward.
• Symmetric Body: A body with a uniform geometric shape and even mass distribution.

Key Points and Lists from the Lecture

• In uniform gravitational fields (like on Earth), CG and CM are at the exact same location.
• CG of a circle/sphere is its center.
• CG of a rectangle is the intersection of its diagonals.
• CG of a triangle is the intersection of its medians.

What Students Should Remember

Actually, Memorizing the location of the Centre of Gravity for different geometric shapes (triangles, squares, cylinders) is a frequent MCQ topic in board exams.

Textbook, Notes, and Practice Links

• Textbook: Physics Full Textbook
• Video Notes: Notes for this Lecture

Final Recap & Board Exam Notes

By identifying the Centre of Gravity, students can simplify complex bodies into single points. This is essential for solving equilibrium problems because gravity's pull is always calculated from the CG.

• Perfect for 9th grade physics board exam prep in Pakistan.*

This notes format is exactly what examiners look for in FBISE 9th class papers.