Explain the principle of stability and the factors (base of support, line of gravity and centre of gravity) th

Concept understanding — Center Of Gravity Balance

The Intuition: The One Point That Holds Everything Together

Think about balancing a pencil on your finger. You slide it back and forth until you find that one spot where it stays perfectly horizontal. That spot isn't random — it's the point where the pencil's weight on the left exactly cancels the weight on the right. If you put your finger anywhere else, the pencil tips.

Now imagine a much more complicated object: a chair, a cricket bat, or even your own body. Every tiny piece of that object is pulled downward by gravity. The center of gravity is the single point where you could pretend all that weight is concentrated. If you support the object exactly at that point, it balances perfectly — no tipping, no rotation.

Note

For most objects on Earth, the center of gravity is the same as the center of mass. The distinction matters only when gravity varies significantly across the object (e.g., a very tall building), which you won't see in school-level problems.

The Precise Statement

The center of gravity of an object is the point at which the entire weight of the object can be considered to act, for the purpose of analyzing translational and rotational equilibrium under gravity.

Mathematically, for a system of particles with weights $w_1, w_2, \dots, w_n$ located at positions $(x_1, y_1, z_1), \dots, (x_n, y_n, z_n)$ , the coordinates of the center of gravity are:

x_{CG} = \frac{\sum w_i x_i}{\sum w_i}, \quad y_{CG} = \frac{\sum w_i y_i}{\sum w_i}, \quad z_{CG} = \frac{\sum w_i z_i}{\sum w_i}

Since weight $w_i = m_i g$ , and $g$ is constant, this simplifies to the center of mass formula:

x_{CM} = \frac{\sum m_i x_i}{\sum m_i}, \quad y_{CM} = \frac{\sum m_i y_i}{\sum m_i}, \quad z_{CM} = \frac{\sum m_i z_i}{\sum m_i}

\vec{R}_{CG} = \frac{\sum m_i \vec{r}_i}{\sum m_i}

Why It Matters …

Start your 14-day free trial to unlock the full solution →

Stability is resistance to losing balance; it increases with a wider base of support, a lower centre of gravity whose line of gravity stays over the base, and greater body mass.

Principle of stability: Stability (balance) is a body's ability to resist being moved or toppled from its position. The more stable the stance, the harder it is to displace — vital in wrestling, judo, gymnastics and defensive stances.

Factors affecting balance:

Base of support — the area beneath and between the body's points of contact with the ground. A wider base (feet apart) gives greater stability.
Line of gravity — the imaginary vertical line running down through the centre of gravity to the ground. Balance is kept only while this line falls within the base of support; if it moves outside the base, the body topples.
Centre of gravity — the point where the body's weight is balanced. A lower centre of gravity (bending the knees, crouching) increases stability. …

Start your 14-day free trial to unlock the full solution →

←Previous questionQ4 — Describe in detail the types of body movements produced at the joints …→Next questionQ5 — A swimmer pushes the water backward and is propelled forward through t…

Q.Explain the principle of stability and the factors (base of support, line of gravity and centre of gravity) that affect balance in a sporting stance.

🎓Unlock everything free for 14 days

Unlock everything free for 14 days