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# Weird question about speed watch

1. I was doing some revision of the thermal stuff and I decided to have a go at the question in the purple box in the attached link but I don't see how part (a) can be worked out if we don't know any time values, can the time values be work out using the graph?

https://www.pearsonschoolsandfecolle...s/Sp_1_3_2.pdf
2. (Original post by Ki Yung Na)
..
If you see lightning then start counting until you hear the thunder, it's quite easy to calculate. Lets say you count 10 seconds between the two. You know this corresponds to around 2 miles. To convert from miles to km, we multiply by roughly 1.6, and a further 1000 to meters.

Hence:

Which is close to the actual value.

3. Convert from M to m to get the speed in ms-1​.
4. (Original post by Phichi)
...
Ah I see, so then whatever the number counted, the fraction will always be 1/5 x conversion?

(Original post by morgan8002)

Convert from M to m to get the speed in ms-1​.
Thanks, I actually did this but calculated some weird number but now I know I didn't convert it!

Thanks guys.
5. awks, thought you wanted drug advice by the title...
6. (Original post by Ki Yung Na)
Ah I see, so then whatever the number counted, the fraction will always be 1/5 x conversion?
Indeed. The distance in miles will always be . If you wanted that in km, it'd just be
7. (Original post by Ki Yung Na)
I was doing some revision of the thermal stuff and I decided to have a go at the question in the purple box in the attached link but I don't see how part (a) can be worked out if we don't know any time values, can the time values be work out using the graph?

https://www.pearsonschoolsandfecolle...s/Sp_1_3_2.pdf

It says the distance in miles can be calculated by counting the seconds from the flash (instant the thunder is generated) until it can be heard and dividing that time in seconds by five to give an answer in miles.

Speed = distance / time = 1 mile / 5 seconds

1 mile = 1600 metres approx.

So approximate speed of sound in air using this method is 1600/5 = 320m/s

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