Coplanar forces, of magnitudes \(F \text{ N}, 3F \text{ N}, G \text{ N}\) and \(50 \text{ N}\), act at a point \(P\), as shown in the diagram.
(i) Given that \(F = 0, G = 75\) and \(\alpha = 60^\circ\), find the magnitude and direction of the resultant force. [4]
(ii) Given instead that \(G = 0\) and the forces are in equilibrium, find the values of \(F\) and \(\alpha\). [5]
Solution
(i) Resolve the forces horizontally and vertically:
Horizontal component: \(X = 75 + 50 \cos 60^\circ = 100\)
Vertical component: \(Y = 50 \sin 60^\circ = 43.3\)
Resultant force: \(\sqrt{100^2 + 43.3^2} = 109 \text{ N}\)
Direction: \(\text{Angle} = \arctan\left(\frac{43.3}{100}\right) = 23.4^\circ\) anticlockwise from the positive x-axis.
(ii) For equilibrium, resolve forces:
Horizontal: \(50 \cos \alpha - F \cos 50^\circ = 0\)
Vertical: \(50 \sin \alpha - 3F - F \sin 50^\circ = 0\)
\(\tan \alpha = \frac{3F + F \sin 50^\circ}{F \cos 50^\circ}\)
Solving gives \(\alpha = 80.3^\circ\) and \(F = 13.1 \text{ N}\).
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