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# CBSE Class 9 Answered

<div>A body moving in &nbsp;a straight line cones a distance of 14m in 5th sec &nbsp;and 20m in 8th sec . how much distance will it cover in 15th &nbsp;sec&nbsp;</div>
Asked by drdilipmane1968 | 29 Jul, 2017, 10:05: PM Expert Answer
This query becomes simple if you consider the distance formula and find u and a from it and solve for the asked time interval.

The following steps will aid you in the query:

1) Consider distance formula, s = ut + at2/2. This gives the total distance travelled in t seconds. But, the given data is of tth second and not total t seconds.

2) So, consider for 5th second first. S5 = distance travelled in 5 seconds - distance travelled in 4 seconds. S5 = [(5u + 25a/2) - (4u + 16a/2)]. You will get S5 = 14 = u + 9a/2.

3) Similarly, find S8 = 20 = u + 15a/2.

4) So, now you have 2 equations. Solve them to get the values of 'u' and 'a'. Once you subtract them, you will get 'a' = 2 m/s2.

5) Then you multiply one equation by 10 and the other by 6. Then subtract the two equations and solve to get 'u'. You will get u = 5 m/s.

6) Finally use the values of u and a to get S15 = [(15u + 225a/2) - (14u + 196a/2)] = u + 29a/2 = 34 m.
Answered by Romal Bhansali | 30 Jul, 2017, 12:32: PM
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