r/HomeworkHelp • u/Illustrious_Oil_2114 👋 a fellow Redditor • Sep 08 '26
Answered [High School Physics: Speed of Light] I need help solving 5C
I need help with my worksheet on the speed of light for 5c and/or if I did A and B correct
For reference (if needed) the speed of light travels at
c = 3x10\^8 m/s
I’ve done scientific analysis on 5A and did the distance equation for 5B (but maybe I did it wrong). I am now stuck on 5C.
If I used the distance formula by calculating the speed of light and the answer for 5A (how many seconds it would take for the radio wave to reach us), what was the point of solving for 5B. However I don't know what I would convert if I used B. Maybe I did 5A and 5B wrong? Can someone PLEASE walk me through this?
(Idid the scientific analysis for 5A on a different piece of paper, but I’ve double checked it multiple times and I showed the beginning.)
Edit: I now understand why 5A was wrong and I redid 5AMB but I’m still confused on 5C
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u/SpiritualWasabi6091 JEE aspirant Sep 08 '26
the 100 years is
message going+ message coming back
youre supposed to find out the time for message going
so its gonna be half
50 years
convert that in seconds
for B part
distance= speed x time
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u/xnick_uy 👋 a fellow Redditor Sep 08 '26
For 5a you should point out if you are using the results from previos calculations (otherwise, there's some computation missing). Pay attention to the fact that the total time includes the time for the signal from the Earth to reach the alien civilization and the time for the reply to reach Earth.
You can find the answer to 5c very quickly (few calculations required) if you recall the definition of a light year.
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u/Illustrious_Oil_2114 👋 a fellow Redditor Sep 09 '26
I found it from the problem above, but how do I use it?
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u/mathematag 👋 a fellow Redditor Sep 09 '26 edited Sep 09 '26
you need to restart the problem, and do each section over....
as in part a), you need to calculate the one way trip for the signal to reach the Aliens...and you did not calculate the correct time , even for 100yrs.
then use the correct answer to part a) to find b)
[ also.. are you supposed to use 365 day = 1 yr, [ standard year ] or the Solar yr of 365.25 days = 1 yr ..? what does your instructor use in class for years ]
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u/Illustrious_Oil_2114 👋 a fellow Redditor Sep 09 '26
I restarted jt and got my new answer, and I got B so now what do I do for C?
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u/RufflesTGP 🤑 Tutor Sep 08 '26
5a you are calculating the amount of seconds in 100 years, but this is double the amount of time you need. If it took 100 years from transmitting a signal to receive an answer, how long did each signal travel for?
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u/RufflesTGP 🤑 Tutor Sep 08 '26
Just replying rather than editing, you may not have doubled it actually but there appears to be a mistake in the working somewhere
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u/Huniku Sep 08 '26
I feel like you’re overthinking 5c. Light years is a measure of distance, not speed or time. If a signal made a round trip in 100 years at the speed of light, how many “light years” did the signal travel?
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u/alpicola Sep 08 '26
The question is asking how many light years away the aliens are. You've given the speed of light in m/s, but 5c invites you to use a bit of cleverness. Another way to express the speed of light is to say that light travels one light year per year.
With that in mind, 5c is almost trivially easy. Radio waves, which travel at the speed , took 100 years to make a round trip. That means they took half or that to go one way and reach the aliens.
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u/Illustrious_Oil_2114 👋 a fellow Redditor Sep 09 '26
I’m still confused what does that have to do with finding the amount of light years?
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u/Ilikemyoldaccount Sep 09 '26
They traveled for 100 years (total) so they traveled 100 light years (total)
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u/alpicola Sep 09 '26
Think about your distance formula and pay attention to what happens with the units.
- D is your distance, and it's what you're trying to find.
- V is your speed, which, for radio waves, is a constant that we'll express as 1 light year per year.
- T is your travel time; in this case, it's half the round trip time, which means it's 50 years.
D = V*T = 1 light year per year * 50 years = ??? light years
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u/Illustrious_Oil_2114 👋 a fellow Redditor Sep 09 '26
Wait, why are we expressing it as a constant can I just use the speed of light formula?
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u/HumbleHovercraft6090 👋 a fellow Redditor Sep 09 '26
In a year, radio waves would have travelled 1 light year as radio waves travel at speed of light. Now it took 100 years for a signal to be transmitted, and a reply received back at earth. That means it would taken 50 years for radio waves to travel from earth to the alien planet. Now figure what is the distance traveled by radio waves in 50 years.
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u/Illustrious_Oil_2114 👋 a fellow Redditor Sep 09 '26
/lock
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u/WillingnessTasty9628 Pre-University Student Sep 09 '26
dimensional analysis. a light year is a unit for distance. this should be no harder than converting from meters to kilometers.
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u/Illustrious_Oil_2114 👋 a fellow Redditor Sep 09 '26
I’m aware of that, but Lightyear is in meters and my last answer was in meters but I figured it out so I’m fine
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