The Balancing Point & the Whirl

Why it matters

Racing cars hug the ground; loaded lorries roll over on bends the same car takes easily. And when a whirling stone breaks its string, almost everyone points the wrong way it flies.

The idea in plain words

Two ideas, one lesson. Tap the terms.

stability: plumb line inside base · circular motion: centripetal pull

Why does a loaded double-decker take corners slower than a race car?

High CG topples early; low CG corners hard

• the balance point: CG — centre of gravity — the point where the whole weight acts; found by the plumb-line method
• stability: topple rule — a body falls over when the vertical through its CG leaves its base — low CG + wide base = stable
• toward centre: centripetal — the REAL inward force that keeps anything on a circle (string tension, gravity, friction)
• the impostor: centrifugal — NOT a real force — just your inertia felt inside the turning frame
• Load upstairs → CG high → plumb line exits the wheelbase at a small lean
• Race car: CG near the road → huge lean needed to topple

Predict first

You whirl a stone on a string. The string SNAPS. The stone flies…

Nothing ever pulled it outward. The string pulled INWARD (centripetal). Remove that pull, and the stone simply keeps its velocity — tangent.

• along the tangent — straight in its current direction — correct
• straight outward from the centre
• it spirals outward gradually

What you do

First topple the roof-loaded truck and save the floor-loaded one. Then place your bet and cut the string.

Check yourself

In uniform circular motion, the speed is constant but the body still accelerates because…

Velocity is speed + direction. Turning = changing velocity = acceleration, supplied by the centripetal force.

The CG of a uniform metre rule is at…

Uniform body → CG at the geometric centre. Hang it anywhere else and gravity's moment swings it back.

A Bunsen burner has a wide, heavy base so that…

Low CG + broad base = large topple angle. Same reason ships carry ballast and racing cars hug the tarmac.

• its DIRECTION (so its velocity) changes every instant — correct
• it doesn't accelerate — speed is constant
• gravity increases on curves
• the 50 cm mark — correct
• the 0 cm end
• it has no CG
• its CG stays low and the plumb line stays inside the base when nudged — correct
• it heats more evenly
• it looks professional