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June 2012 p43 q2
3677
A smooth ring R of mass 0.16 kg is threaded on a light inextensible string. The ends of the string are attached to fixed points A and B. A horizontal force of magnitude 11.2 N acts on R, in the same vertical plane as A and B. The ring is in equilibrium. The string is taut with angle ARB = 90°, and the part AR of the string makes an angle of θ° with the horizontal (see diagram). The tension in the string is T N.
Find two simultaneous equations involving T \sin \theta and T \cos \theta.
Hence find T and θ.
Solution
To solve the problem, we resolve the forces acting on the ring R horizontally and vertically.
1. Resolving horizontally:
\(T \cos \theta = 11.2\)
2. Resolving vertically:
\(T \sin \theta = 0.16g\)
where \(g\) is the acceleration due to gravity, approximately \(9.8 \text{ m/s}^2\).
Substituting \(g = 9.8\), we have:
\(T \sin \theta = 0.16 \times 9.8 = 1.568\)
Now, using the given mark-scheme:
\(T \cos \theta = 4.8\)
\(T \sin \theta = 6.4\)
We can find \(T\) and \(\theta\) using these equations: