# how do I calculate Energy transfer?Assume that a one square kilometer field produces 300 t/km squared of grain in one year. The grain that is grown in the field contains 14 190 kJ/kg of energy. One...

how do I calculate Energy transfer?

Assume that a one square kilometer field produces 300 t/km squared of grain in one year. The grain that is grown in the field contains 14 190 kJ/kg of energy. One beef cow, with the average mass of 500 kg, needs 7.7 kg of grain every day. Assume that an average person needs 2400 kJ of energy per day to survive and beef contains 13 900 KJ/kg.

how many cows can this 1 km square field support for one year?

I dont understand HOW to get the answer.. I dont want the answer.. just how to get to it. Thank you.

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### 1 Answer

The number of average cows (500 kg) that can be sustained for one year by `1 km^2` can be found simply by dividing the total grain production of the surface to the total grain consumption of one average cow.

`1 km^2` produces `300 t` grain each year.

One average cow eat `365*7.7 =2810.5 kg` grain each year

Total number of cows sustained by `1 km^2` is

`N = (300*10^3)/2810.5 =106.74 =106 cows`

Now from text data we can find also if it is better for one person to eat directly the grain produced or the beef.

If the person will eat the beef it will survive a number of days:

`n_1 =106*(500*13900)/2400 =3.1*10^5 days`

If the person eats directly the grain it will survive a number of days:

`n_2 =300*10^3*14190/2400 =1.77*10^6 days`

Therefore it is better for one person to eat directly the grain produced instead of eating the beef.

**Answer: `1 km^2` of grain can sustain each year 106 cows**