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A light spring balance is suspended from the hook of another light spring balance, and a block of mass M kg is attached to the lower balance. Which statement correctly describes the readings of the two balances?

1. Each balance shows M kg.
2. The lower balance shows M kg, while the upper balance shows zero.
3. The two readings may take any values, but their total is M kg.
4. Each balance shows M/2 kg.

Correct answer: Each balance shows M kg.

Solution

The whole weight Mg is transmitted through both spring balances in the series chain (each is light, so it adds no weight). Tension is Mg everywhere, so each balance reads M kg.

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