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The maximum speed in m/s at which the 8th bright fringe will move is .
24.00 to 24.15
y8 = 8 Dd | dy8dt | = 8 Dd2 | d(d)dt |, with d(d)dt = 0.04 10-3 0.08 t = 3.2 10-6 t m/s.
At the mean position ( t = 1, d = 0.8 mm): v = 4.8 10-6 3.2 10-6(0.8 10-3)2 = 2.4 10-5 m/s = 24 m/s.
(Maximising td2 exactly shifts the peak slightly to t -0.1, giving 24.12 m/s; to the intended accuracy v 24 m/s.)
Answer: 24.00
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