4 A lorry of mass 12000 kg moves up a straight hill of length 500 m , starting at the bottom with a speed of \(24 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) and reaching the top with a speed of \(16 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). The top of the hill is 25 m above the level of the bottom of the hill. The resistance to motion of the lorry is 7500 N . Find the driving force of the lorry.
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\caption{Fig. 1}
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Four coplanar forces of magnitudes \(4 \mathrm {~N} , 8 \mathrm {~N} , 12 \mathrm {~N}\) and 16 N act at a point. The directions in which the forces act are shown in Fig. 1.
- Find the magnitude and direction of the resultant of the four forces.
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\caption{Fig. 2}
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The forces of magnitudes 4 N and 16 N exchange their directions and the forces of magnitudes 8 N and 12 N also exchange their directions (see Fig. 2). - State the magnitude and direction of the resultant of the four forces in Fig. 2.