To find the energy lost due to air friction, we apply the Law of Conservation of Energy, which states that the total initial energy of the body must equal the total final energy plus any energy dissipated by non-conservative forces like friction .

1. Initial Kinetic Energy ($KE_i$): $KE_i = \frac{1}{2}mv^2 = \frac{1}{2} \times 1 \text{ kg} \times (20 \text{ ms}^{-1})^2 = 200 \text{ J}$
2. Final Potential Energy ($PE_f$) at 18 m: $PE_f = mgh = 1 \text{ kg} \times 10 \text{ ms}^{-2} \times 18 \text{ m} = 180 \text{ J}$
3. Energy Balance: Initially, the body has 200 J of energy. At its highest point, it only possesses 180 J of potential energy (since its velocity, and thus its kinetic energy, is zero) . $\text{Energy Lost} = KE_{initial} - PE_{final}$ $\text{Energy Lost} = 200 \text{ J} - 180 \text{ J} = 20 \text{ J}$

Thus, 20 J of energy was dissipated as heat due to air friction .