Answer
To pull the cart, the horse needs to eat 433 grams of oats.
Work Step by Step
We can find the work $W_h$ done by the horse on the cart:
$Work = \Delta KE$
$W_h-F_f~d = \frac{1}{2}mv^2$
$W_h = \frac{1}{2}mv^2+F_f~d$
$W_h = \frac{1}{2}(250~kg)(0.38~m/s)^2+(260~N)(1500~m)$
$W_h = 3.9\times 10^5~J$
This work is only 10% of the energy that the horse gets from eating oats. Therefore, to pull the cart, the horse needs to eat enough oats to get $3.9\times 10^6~J$ of energy.
We can find the number of grams of oats that the horse needs to eat:
$\frac{3.9\times 10^6~J}{9000~J/gram} = 433~grams$
To pull the cart, the horse needs to eat 433 grams of oats.
