Centre of mass
The balance point of an object, where its weight effectively acts; the object topples once that point moves beyond its base.
The centre of gravity is the point in an object where its whole weight can be thought of as acting. In everyday conditions it is the same as the centre of mass, the average position of all the object's mass. For a hammer it is close to the heavy head.
Whether something stands or falls depends on where that point sits relative to the base it rests on. Imagine a line straight down from the centre of gravity. If the line falls inside the base, the weight pulls the object back upright; if it falls outside, the weight swings it over and it topples. Objects with a low centre of gravity and a wide base are the most stable, which is why racing cars are low and broad and a tall, narrow vase is easy to knock over.
The Leaning Tower of Pisa shows how far the rule can be pushed. By 1990 its tilt had reached 5.5 degrees and it was closed. Engineers removed soil from under the raised side and added lead counterweights, bringing it back to just under 4 degrees.
The rule can also be used deliberately. Tip a heavy upright object onto one edge of its base and its weight balances over that edge; rock it from edge to edge, twisting a little each time, and it "walks" forward. Removal men shuffle fridges this way, and it may be how the great statues of Easter Island were moved.
Coming up in future stories
- Easter Island's Statues Walked c. 1200–1600 · Rapa Nui (Easter Island), Chile
- Fosbury's Backwards Leap and the Hidden Centre of Mass 20 Oct 1968 · Mexico City (developed 1963-64 · Medford, Oregon)
- Lindbergh Flew to Paris Without a Front Window 20–21 May 1927 · New York to Paris
- The Throw So Long They Redesigned the Javelin 20 Jul 1984 · Berlin, East Germany / 1986 rule change
- The Warship That Sank 1,300 Metres Into Its Voyage 10 August 1628 · Stockholm harbour, Sweden