Motion in a Straight Line - Result Question 8
8. A car covers the first half of the distance between two places at $40 km / h$ and other half at $60 km / h$. The average speed of the car is
(a) $40 km / h$
(b) $48 km / h$
(c) $50 km / h$
(d) $60 km / h$
[1990]
Topic 2: Non-uniform motion
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Answer:
Correct Answer: 8. (b)
Solution:
- (b) Total distance $=s$;
Total time taken
$=\frac{s / 2}{40}+\frac{s / 2}{60}=\frac{5 s}{240}=\frac{s}{48}$
$\therefore$ Average speed $=\frac{\text{ total distance }}{\text{ total time }}$
$=\frac{s}{s / 48}=48 km / h$
$v _{a v}=\frac{2 v_1 v_2}{v_1+v_2}=\frac{2 \times 40 \times 60}{40+60}=48 km / h$
