Physics (0625) May June 2011 Paper 31- Doubt

[SebastianM]

Question Number 7(b)(ii)
The shock wave from the earthquake travels at 2.5x10^3 m/s.
The centre of the earthquake is 6.0x10^5 m from the coast of a country.
Calculate how much warning of the arrival of the tsunami at the coast is given by the earth tremor felt at the coast.

 

[SebastianM]

u use the speed u calculated in the part before and u calculate the time for both to reach and then minus them from each other.... and that will be the time...

I have tried many ways, but I can not find the answer.
If you would be kind enough to show me with the workings, it would really be great.

 

[SebastianM]

speed of the tsunami wave... is 240 m/s but the speed of the shockwave is 2.5x10^3 so shockwave is faster and reaches first!
so the distance is 6.0 x 10^5. then u use the equation.. time=distance/speed so ... 6.0x10^5/240 and 6.0x10^5/2.5x10^3 so the first time is 2500 seconds and the second one is 240 seconds so the time for warning in seconds is 2260... as u minus them... but to make it easier convert to minutes by 2260/60 = 37.6666666667 then take the 37 as minutes and multiply the 0.66666666667x60 u will get 40 seconds so... WARNING TIME IS 37 Minutes And 40 Seconds!! EASY.......

Thanks a tonne! I finally understand now!