June 2012 p41 q2
3694
Forces of magnitudes 13 N and 14 N act at a point O in the directions shown in the diagram. The resultant of these forces has magnitude 15 N. Find
- the value of \(\theta\),
- the component of the resultant in the direction of the force of magnitude 14 N.
Solution
(i) Using the cosine rule in the triangle formed by the forces:
\(X = 14 - 13 \cos \theta\)
\(Y = 13 \sin \theta\)
\(14^2 + 13^2 - 2 \times 13 \times 14 \cos \theta = 15^2\)
Solving for \(\theta\), we find \(\theta = 67.4^\circ\).
(ii) The component of the resultant in the direction of the 14 N force is:
\(X = 15 \cos \left( \arctan \left( \frac{Y}{X} \right) \right)\)
The component is 9 N.
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