CIE IGCSE Physics (0625) Centre of gravity Study Notes - New Syllabus
CIE IGCSE Physics (0625) Centre of gravity Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Centre of gravity
Key Concepts:
- Centre of gravity
- Experiment: Finding the Centre of Gravity of an Irregular Lamina
- Centre of Gravity and Stability
Centre of Gravity
Centre of Gravity:
- The centre of gravity (CoG) of an object is the point at which the entire weight of the object may be considered to act.
- It is also referred to as the centre of mass when the gravitational field is uniform.
- The position of the centre of gravity depends on the shape and distribution of mass within the object.
Key Points:
- For a regular, symmetrical object with uniform density, the centre of gravity lies at the geometric centre (e.g., the center of a square or circle).
- For an irregular object, the centre of gravity may be found by experiment (using plumb line method).
- If the object is suspended freely from its centre of gravity, it will balance perfectly and remain at rest.
Example:
Where is the centre of gravity of a uniform 1 metre wooden ruler?
▶️ Answer/Explanation
Since the ruler is uniform and symmetrical, its weight is evenly distributed.
The centre of gravity is at the 50 cm mark, which is its midpoint.
Experiment: Finding the Centre of Gravity of an Irregular Lamina
Experiment: Finding the Centre of Gravity of an Irregular Lamina
- Objective: To find the point through which the entire weight of an irregularly shaped, flat object (lamina) acts – its centre of gravity.
Apparatus:
- Irregular cardboard or plastic lamina (with holes near the edge)
- Clamp stand with a boss and pin or nail
- Plumb line (string with small weight)
- Pencil and ruler
Procedure:
- Hang the lamina freely from a hole near its edge using a pin or nail through the clamp stand.
- Allow it to come to rest.
- Hang the plumb line from the same pin in front of the lamina.
- When both are steady, use a pencil and ruler to draw a vertical line on the lamina that follows the string of the plumb line.
- Repeat the above steps from two other holes along the edge of the lamina.
- The point where all the lines intersect is the centre of gravity.
Conclusion:
- The point of intersection of all the vertical lines is the only point where the lamina can be balanced horizontally.
- This is the centre of gravity of the irregular shape.
Example:
A student cuts out an irregularly shaped cardboard piece and follows the above procedure. The three vertical lines drawn from different suspension points intersect at one spot inside the lamina. What does this tell you?
▶️ Answer/Explanation
This confirms that the intersecting point is the centre of gravity of the lamina.
If the student balances the lamina on a pin at that exact point, it will remain balanced.
Centre of Gravity and Stability
Centre of Gravity and Stability:
- The stability of an object refers to how likely it is to return to its original position after being slightly disturbed (tilted).
- The position of the centre of gravity plays a major role in determining this stability.
Key Relationships:
- Lower Centre of Gravity → Greater Stability
- The object is less likely to topple over.
- A wider base combined with a low centre of gravity improves balance (e.g., sports cars, wide vases).
- Higher Centre of Gravity → Less Stability
- The object becomes top-heavy and more likely to topple when tilted.
- Objects like double-decker buses and tall shelves are less stable and must be carefully designed.
- Narrow Base and High Centre of Gravity → Very Unstable
- A small disturbance can cause the vertical line through the centre of gravity to fall outside the base, leading to toppling.
Visual Rule:
If the vertical line from the centre of gravity falls within the base, the object will return to its original position → Stable.
If the vertical line from the centre of gravity falls outside the base, the object will topple → Unstable.
Example:
Compare the stability of a cone standing on its base and the same cone standing on its tip.
▶️ Answer/Explanation
Cone on base: Has a low centre of gravity and wide base → Very stable.
Cone on tip: High centre of gravity and narrow base → Very unstable.
Small disturbances will cause the cone to fall when standing on its tip.