First Class Question

[hazey]

I was having a go at one of the first class papers but I am having difficulty getting my answer to tally up with the published one. So, without looking, what is the answer to the following:

You need to raise a steel object approximating a volume of 0.005m^3 (meter cubed) (Density 8000kg/m^3) from 40m. How much air is needed to lift the object?

Assume density of sea water is 1.025 kg/litre

Thanks

[PeteM]

I was having a go at one of the first class papers but I am having difficulty getting my answer to tally up with the published one. So, without looking, what is the answer to the following:

You need to raise a steel object approximating a volume of 0.005m^3 (meter cubed) (Density 8000kg/m^3) from 40m. How much air is needed to lift the object?

Assume density of sea water is 1.025 kg/litre

0.005 * 8000 = 40Kg mass

Upthrust (from Archimedes Principle) = 0.005 * 1.025 * 1000 = 5.125Kg

Effective weight = 40 - 5.125Kg = 34.875

From Archimedes Principal again we need to displace enough water to weigh that amount so lift required = 34.875 / 1.025 = 34.024 Litres

40m = 5 bar so you need 170.12 L of air

Unless I've made a mistake ;-)

[hazey]

That makes sence to me. I guess the answer on the web site is wrong then.

[Paul Leyland]

I was having a go at one of the first class papers but I am having difficulty getting my answer to tally up with the published one. So, without looking, what is the answer to the following:

You need to raise a steel object approximating a volume of 0.005m^3 (meter cubed) (Density 8000kg/m^3) from 40m. How much air is needed to lift the object?

Assume density of sea water is 1.025 kg/litre

Does the question go on to ask how you could make it safer, and state that you have a lot of rope and oil drums available?

If so, it was one of the questions on my paper when I passed!
The answer given by Pete seems correct to me. But, use the ropes to lift it 10m and tow into shallower water, then lift again.

Good luck with the exam!
Paul First Class Trainee no FC702

[PeteM]

That makes sence to me. I guess the answer on the web site is wrong then.

Looking at the answer I think they have neglected to convert the object volume from cubic meters to litres

[john bache]

Well ... it they are going to be really picky .... at 40m in sea water the pressure isnt 5 bar ... its more like 4.878 Bar ... making the air needed to give neutral buoyancy to the thingy as 165.9726 Litres

And before anyone says anything ... Yes I'm sure that 4 decimal places are neccesary when filling a lift bag at 40 Meters.

Doh !

Good luck with your exam,

John.