
Question 18

 A uniform bar PQ of length 60 cm and weight 20 N is supported at two points C and D such that PC = 10 cm and QD = 15 cm. Two forces 10 N and 15 N are placed at P and Q respectively. If the system remains in equilibrium under the action of these forces, calculate the reactions at C and D. [Take g = 10 ms2]
 Two forces 3i N and 4j N act on an object of mass 5 kg. Find the acceleration of the object.

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It was reported that this question was poorly attempted by majority of the candidates. According to the report, their responses showed lack of understanding of the topic as majority of them could not represent the information in part (a) on a diagram. Neither could they take the moments correctly. WAEC was encouraged to set questions on this topic so that teachers as well as candidates could be encouraged to study the topic.
In part (a), candidates were expected to respond as follows:
Let R1 and R2 be the reactions at C and D with all other forces and directions as shown in the diagram
Taking moments at D, 35R1 + (15 x 15) = (20 x 15) + (10 x 45). i.e. 35R1 = 750 – 225 = 525. Hence, R1 = 15N. R1 + R2 = 45 N. This implied that R2 = 45 – R1 = 30 N.
In part (b), F1 = 3i N, F2 = 4j N. Resultant force, F = 3i + 4j. Magnitude of resultant force = F =
= = 5 N. Using the relation F = ma, where F= 5 N and m = 5 kg, a = = 1 ms2.
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