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5.

A body falling freely from the top of a tower crosses 49\frac{4}{9} of the tower's height in the last second. What is the height of the tower?

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AI-তৈরি নমুনা উত্তর

Let the height of the tower be hh and the total time taken to reach the ground be tt seconds.
Since the body falls freely from rest (u=0u = 0):
h=12gt2h = \frac{1}{2}gt^2

The distance traveled in the last second (i.e., the tt-th second) is:
hlast=12g(2t−1)h_{\text{last}} = \frac{1}{2}g(2t - 1)

According to the question:
hlast=49hh_{\text{last}} = \frac{4}{9}h
12g(2t−1)=49(12gt2)\frac{1}{2}g(2t - 1) = \frac{4}{9}\left(\frac{1}{2}gt^2\right)
2t−1=49t22t - 1 = \frac{4}{9}t^2
4t2−18t+9=04t^2 - 18t + 9 = 0

Solving for tt using the quadratic formula:
t=−(−18)±(−18)2−4(4)(9)2(4)=18±324−1448=18±1808=9±354t = \frac{-(-18) \pm \sqrt{(-18)^2 - 4(4)(9)}}{2(4)} = \frac{18 \pm \sqrt{324 - 144}}{8} = \frac{18 \pm \sqrt{180}}{8} = \frac{9 \pm 3\sqrt{5}}{4}

Since the fall takes at least 1 second (t>1t > 1):
t=9+354≈3.927 st = \frac{9 + 3\sqrt{5}}{4} \approx 3.927 \text{ s}
(The other root t≈0.573 s<1t \approx 0.573\text{ s} < 1, which is inadmissible).

Taking g=9.8 m/s2g = 9.8 \text{ m/s}^2:
h=12gt2=12×9.8×(3.927)2≈75.56 mh = \frac{1}{2}gt^2 = \frac{1}{2} \times 9.8 \times (3.927)^2 \approx 75.56 \text{ m}

(Note: If the fraction in the standard textbook problem is 59\frac{5}{9} instead of 49\frac{4}{9}, then 5t2−18t+9=0  ⟹  (t−3)(5t−3)=0  ⟹  t=3 s5t^2 - 18t + 9 = 0 \implies (t - 3)(5t - 3) = 0 \implies t = 3\text{ s}, which gives h=12×9.8×32=44.1 mh = \frac{1}{2} \times 9.8 \times 3^2 = 44.1\text{ m}.)

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