TBR physics error? (easy question)

[ilovemedi]

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If deep-water ocean waves elevate a stationary boat
every 4 seconds and the distance between every third
crest is 150 m, what is the average speed of the waves?
A. 0.08 m/s
B. 12.5 m/s
C. 37.5 m/s
D. 113 m/s
Answer: B

My reasoning: Should be 18.5m/s

Crest-----------Crest-----------Crest-----------Crest
--------lambda 1-----lambda 2------lambda 3

isn't the wavelength 75 between each crests? because there's 2 wavelengths when connecting the 3 crests (as pictured above), thus wavelength os 150cm/2 = 75...
And v=1/T*wavelength. Thus v=1/4*75

 

[Chrisz]

You are correct, it is indeed a mistake. Each cycle takes 4 seconds. The distance between the first crest and the third crest is actually the distance of having finished two complete cycles. So wavelength is just 0.5 of 150m. So, 150/2/4= 18.75m/s. Dont give too much attention to it as along as you understand and are confident about what you doing.

 

[SOOyC52H1Sof]

yep. i got the same thing

T=4 s
λ=75m

so then ƒ=1/T= 1/4

v=λƒ = 75*¼ = 18.75

 

[ilovemedi]

Thanks guys!

 

[DrknoSDN]

I got the right answer of 12.5 m/s
I think it's the wording of the problem.

Think about this,,, if your organizing a sport in PE class or whatever and you pick every third person for your team... you will pick person 1, 4, 7 etc.. So the distance between (Crest) 1 and 4 is 150 meters...

So 3 full periods is 150 meters. 50 meters per period. and 4 seconds between periods. = 12.5 m/s

It's the wording of the problem,,, not your misunderstanding of the concept.

 

[Chrisz]

I got the right answer of 12.5 m/s
I think it's the wording of the problem.

Think about this,,, if your organizing a sport in PE class or whatever and you pick every third person for your team... you will pick person 1, 4, 7 etc.. So the distance between (Crest) 1 and 4 is 150 meters...

So 3 full periods is 150 meters. 50 meters per period. and 4 seconds between periods. = 12.5 m/s

It's the wording of the problem,,, not your misunderstanding of the concept.

Thanks man, totally misunderstood the wording. every third......lol, should have played some more sports.

 

[BerkReviewTeach]

I got the right answer of 12.5 m/s
I think it's the wording of the problem.

Think about this,,, if your organizing a sport in PE class or whatever and you pick every third person for your team... you will pick person 1, 4, 7 etc.. So the distance between (Crest) 1 and 4 is 150 meters...

So 3 full periods is 150 meters. 50 meters per period. and 4 seconds between periods. = 12.5 m/s

It's the wording of the problem,,, not your misunderstanding of the concept.

PERFECT! Like every other is the 1st, 3rd, 5th, etc..., every third is the 1st, 4th, 7th, etc... This is a bs question intentionally included in the books to expose people to bs wording. It's unfortunately a part of the training for this exam, because there will very likely be a few questions on the actual exam that have some bs wording.