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Nov 2013 p43 q2
3943
Particle A of mass 0.2 kg and particle B of mass 0.6 kg are attached to the ends of a light inextensible string. The string passes over a fixed smooth pulley. B is held at rest at a height of 1.6 m above the floor. A hangs freely at a height of h m above the floor. Both straight parts of the string are vertical (see diagram). B is released and both particles start to move. When B reaches the floor it remains at rest, but A continues to move vertically upwards until it reaches a height of 3 m above the floor. Find the speed of B immediately before it hits the floor, and hence find the value of h.
Solution
Let the acceleration of the system be \(a\). Using Newton's second law for both particles:
For particle A: \(T - 0.2g = 0.2a\)
For particle B: \(0.6g - T = 0.6a\)
Adding these equations gives:
\(0.6g - 0.2g = 0.6a + 0.2a\)
\(0.4g = 0.8a\)
\(a = \frac{0.4g}{0.8} = 5 \, \text{m/s}^2\)
When B reaches the floor, using \(v^2 = u^2 + 2as\) with \(u = 0\), \(a = 5\), and \(s = 1.6\):
\(v^2 = 2 \times 5 \times 1.6\)
\(v^2 = 16\)
\(v = 4 \, \text{m/s}\)
For A to reach 3 m above the floor, the total distance moved by A is \(3 - h\). Using energy conservation: