Lecture 4a: Centre of Gravity and Weight

Vocabulary-style flashcards covering the definitions and principles of gravity, mass, weight, and the centre of gravity as they relate to human movement and physiotherapy.

Gravity

A natural force that attracts all physical objects towards each other; on Earth, it causes things to fall towards the ground.

Acceleration due to gravity (g)

A constant for objects relatively close to the Earth’s surface, approximately equal to $9.8\,ms^{-2}$.

Fall Statistics (Adults 65+)

The leading cause of disease burden in this demographic, with over one-third falling annually and $15\%$ of those falls leading to injury.

Mass

The quantity of the physical property of matter, measured in kilograms ($kg$).

Weight

The result of object mass ($kg$) multiplied by acceleration of gravity ($9.8\,ms^{-2}$), measured in Newtons ($N$).

Weight Formula

$$Weight (N) = mg$$

Acceleration due to gravity (g) on the moon

$$1.62\,ms^{-2}$$

Centre of mass

The mean position of the mass of the object; it does NOT change or alter with the orientation of the body.

Centre of gravity

The point about which the body can be balanced perfectly against gravity (equilibrium); it can shift depending on orientation and lie inside or outside the body.

Standard Anatomical Location of COG

Anterior to $S2$.

Stability Rule (COG)

The lower the centre of gravity, the more stable an object or body is.