Question
A block of mass 2.0 kg is suspended from the ceiling by a massless string. The string is taut bearing a definite tension when the block is stationary. Suddenly, the string breaks and the block start falling freely. The difference in the tension in string just before and after it breaks is
Options
Solution
Sol. Key concept: Tension before and after a supporting string breaks.
Before the break, the stationary block is in vertical equilibrium, so the upward tension balances its weight. After the string breaks it cannot exert a tensile force; the block then falls freely.
$$T_{before} = mg = 2.0 \times 9.8 = 19.6\ N$$
$$T_{after} = 0,\ \ \ \ \Delta\ T = 19.6 - 0 = 19.6\ N$$
The weight remains mg throughout; it is the tension that becomes zero. Hence option (b).
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