Centre of Gravity
Definition
The centre of gravity (CG) is the point at which the resultant weight of a body (or system of particles) acts — i.e. the theoretical point where all the body’s weight can be considered to be concentrated.
Note: The CG does not have to lie inside the material of the body. Examples: a tyre (ring), a football helmet; even human body positions (e.g. a high-jumper in flight) can have CG outside the body.
Finding the CG of an Irregular Lamina (Plumb-line Method)
- Punch three small holes near different edges of the lamina.
- Suspend the lamina from the first hole on a stand. Hang a plumb-line close to that hole (without touching the lamina).
- Let the system come to rest; draw a line on the lamina along the plumb-line.
- Repeat the suspension from the second and third holes; draw the second and third vertical lines.
- The point where the lines intersect is the centre of gravity (G).
Keep the plumb-line clear of the lamina and wait until it is completely at rest before marking each line.
Stability & the Position of CG
- An object will topple when the vertical line through its CG falls outside its base of support.
- A lower CG → greater stability (harder to topple).
- Example: The Leaning Tower of Pisa doesn’t topple because its CG remains vertically above the base.
Quick Checks
- Q: Why do racing cars have very low chassis?
A: Lower CG increases stability when cornering. - Q: A cabinet is more stable with a wider base. Why?
A: The base of support is larger, so the CG line stays within the base for larger tilts.