# The speed of water moving through a turbine is 2.5 m/s. Show that the mass of water passing through an area of 500 metres squared in one second is about 1 x 10^6 kg (density of sea water = 1030 kg/m^3)

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This is a past exam question from an A level paper for OCR Physics B.

We know that in one second, a volume of water (V), travelling at 2.5 m/s is passing through an area of 500 metres squared in one second. This volume can be represented as a column, with the cross section (area at the front) equal to the area the water is passing through, so 500 metres squared. Since we know that v=s/t, we can rearrange this to get s=vt meaning that in one second, all the water molecules travel v.t metres of 2.5 x 1 = 2.5 metres. This gives us our bottom side for our column, giving us a total volume of V=Al = 500 x 2.5 = 1250 metres cubed.

So we now have the volume (V) and the density (ρ) but want to find the mass (m) which are all linked in the equation ρ=m/V which when rearranged gives m=ρV giving us an answer of m=1030 x 1250 = 1,287,500 kg which we can say is roughly equal to 1 x 10^6 kg

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