Step 1: Write the efficiency formula of a Carnot engine.
For a Carnot engine,
\[
\eta = 1-\frac{T_C}{T_H}
\]
where \(T_H\) is the temperature of the hot reservoir and \(T_C\) is the temperature of the cold reservoir.
Here,
\[
T_H=600\,K
\]
and
\[
T_C=300\,K
\]
Step 2: Calculate the efficiency.
Substituting the values,
\[
\eta = 1-\frac{300}{600}
\]
\[
\eta = 1-\frac{1}{2}
\]
\[
\eta = \frac{1}{2}
\]
So,
\[
\eta = 0.5
\]
Step 3: Use the relation between efficiency and work done.
Efficiency is given by
\[
\eta = \frac{W}{Q_H}
\]
where \(W\) is the work done and \(Q_H\) is the heat absorbed from the source.
Given,
\[
Q_H=800\,J
\]
Therefore,
\[
W=\eta Q_H
\]
\[
W=0.5 \times 800
\]
\[
W=400\,J
\]
Step 4: Final conclusion.
Hence, the mechanical work done per cycle is
\[
\boxed{400\,J}
\]