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Nov 2015 p43 q4
3938
Particles A and B, of masses 0.35 kg and 0.15 kg respectively, are attached to the ends of a light inextensible string which passes over a fixed smooth pulley. The system is at rest with B held on the horizontal floor, the string taut and its straight parts vertical. A is at a height of 1.6 m above the floor (see diagram). B is released and the system begins to move; B does not reach the pulley. Find
the acceleration of the particles and the tension in the string before A reaches the floor,
the greatest height above the floor reached by B.
Solution
(i) Applying Newton's second law to particle A:
\(0.35g - T = 0.35a\)
For particle B:
\(T - 0.15g = 0.15a\)
Adding the two equations:
\((0.35 - 0.15)g = (0.35 + 0.15)a\)
Solving for acceleration \(a\):
\(0.2g = 0.5a\)
\(a = \frac{0.2g}{0.5} = 4\, \text{m/s}^2\)
Substituting \(a\) back to find tension \(T\):
\(T = 0.15g + 0.15 \times 4 = 2.1\, \text{N}\)
(ii) Using the equation \(v_1^2 = 0 + 2a \times 1.6\):