Oh No! Physics exam tomorrow, Zebrahead needs help again!

This is the last time, after tomorrow, physics is all gone! ^^

This time its about maximum potential energy, the question is as follows:

Tidal power could make a significant contribution to UK energy requirements. This question is about a tidal power station which traps sea water behind a tidal barrier at high tide and then releases the water through turbines 10.0 m below the high tide mark.

Calculate the mass of sea water covering an area of 120 km^2 and depth 10.0 m.

(i)
density of sea water = 1100 kg m^–3

(ii) Calculate the maximum loss of potential energy of the sea water in part (i) when it is released through the turbines.

I've done the first part right and got the mass to be 1.32x10^12 kg,

The next part I did Ep = mgh = (1.32x10^12) x 9.81 x 10

As the water drops 10m??

The mark scheme says the deltaH = 5m giving them a energies half that of mine!

Any help would be appreciated, the only thing i can think of is whether the mass is centred half way up the body of water? I really cant think of any other way it's 5m!

Thanks all ^^

Reply 1

Zebrahead

OP

Bump for urgency!

Reply 2

Ry4n11

Yep, the average water particle will fall 5m. The other answers seem to be good.

The water at the top of the barrier will fall 10m, and the water at the bottom of the barrier won't fall at all. Which will average out of 5.

Reply 3

jerzydee

i don't really understand how the 5m comes about. i think the markscheme is wrong.

Reply 4

Smith987

Basically treat the mass of water as acting from the centre of gravity, it will be 5m above the ground and so it will fall 5m :smile:

Reply 5

SSshrinky

Yeah, I hated this question too...

I think it is as simple as 'take the average height'

so 10/2 = 5m :smile:

Reply 6

-WhySoSerious?

17

jerzydee

i don't really understand how the 5m comes about. i think the markscheme is wrong.

See this thread: http://www.thestudentroom.co.uk/showthread.php?t=923721&page=2

The bottom post, and it has a bit more on the exam as well.

Reply 7

wizz_kid

16

-WhySoSerious?

See this thread: http://www.thestudentroom.co.uk/showthread.php?t=923721&page=2

The bottom post, and it has a bit more on the exam as well.

is this q from edexcel physics?

Reply 8

-WhySoSerious?

17

wizz_kid

is this q from edexcel physics?

Nope, AQA A

Reply 9

Francis Y

Ah at least this exam will be in the afternoon.

I'll memorize some mark schemes beforehand. Worked for P5.

Reply 10

101helpme

2

Hi. I'm stuck on the first part of this question lol. The mark scheme says the area is 120 x 10^6 m^2.Im so confused because in the question it says its covering an area of 120km^2 so wouldn't this be 120 x 10^3 m^2 ? Im sure 1km is 1000m?

Reply 11

Joinedup

20

Original post by 101helpme

Hi. I'm stuck on the first part of this question lol. The mark scheme says the area is 120 x 10^6 m^2.Im so confused because in the question it says its covering an area of 120km^2 so wouldn't this be 120 x 10^3 m^2 ? Im sure 1km is 1000m?

think about 1km² as a square with 1km sides
each side length is 1000m
so the area in m² is 1000x1000 = 1x10⁶
i.e. you have 1000 rows of meter squares and each row has 1000 meter squares in it

also thread is ancient - I'm not especially bothered but a lot of people will tell you off for replying to an old thread.

Reply 12

101helpme

2

Original post by Joinedup

think about 1km² as a square with 1km sides
each side length is 1000m
so the area in m² is 1000x1000 = 1x10⁶
i.e. you have 1000 rows of meter squares and each row has 1000 meter squares in it

also thread is ancient - I'm not especially bothered but a lot of people will tell you off for replying to an old thread.

THANK YOU!!! I have been trying to figure it out for 2 days in a row but couldn't get my head around it :h:

Only after I submitted the question had I realised how old the post was and decided to create anew one. Thank you very much for replying

Reply 13

Fohayy

2

Original post by Smith987

Basically treat the mass of water as acting from the centre of gravity, it will be 5m above the ground and so it will fall 5m :smile:

so we just assume that the center of gravity of the water after becoming at sea level at the surface? and now a further 5 m below the new level

Reply 14

Callicious

22

Original post by Fohayy

so we just assume that the center of gravity of the water after becoming at sea level at the surface? and now a further 5 m below the new level

Zebrahead has finished their AS/A2, finished their undergrad, and probably a PhD by now... :_:

Oh No! Physics exam tomorrow, Zebrahead needs help again!

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