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June 2003 p4 q5
3933
O
S1
A
S2
B
S1 and S2 are light inextensible strings, and A and B are particles each of mass 0.2 kg. Particle A is suspended from a fixed point O by the string S1, and particle B is suspended from A by the string S2. The particles hang in equilibrium as shown in the diagram.
(i) Find the tensions in S1 and S2.
The string S1 is cut and the particles fall. The air resistance acting on A is 0.4 N and the air resistance acting on B is 0.2 N.
(ii) Find the acceleration of the particles and the tension in S2.
Solution
(i) In equilibrium, the forces on each particle must balance. For particle A, the tension in S1 must balance the weight of A and the tension in S2. Therefore, we have:
\(T_1 = W_A + T_2\)
For particle B, the tension in S2 must balance the weight of B:
\(T_2 = W_B\)
Given that the weight of each particle is \(W = mg = 0.2 \times 9.8 = 1.96 \text{ N}\), we find:
\(T_2 = 1.96 \approx 2 \text{ N}\)
Substituting back, we find:
\(T_1 = 1.96 + 1.96 = 3.92 \approx 4 \text{ N}\)
(ii) When S1 is cut, both particles fall with acceleration. Using Newton's second law for particle A: