Given data

• Mass of bus: $m = 500\,\text{kg}$
• Speed is constant: $80\,\text{km/h}$
• Distance traveled: $s = 4\,\text{km} = 4000\,\text{m}$
• Coefficient of friction: $\mu = 0.04$

Key idea

Since the bus moves on a plane rough road with constant speed, its acceleration is zero.

Therefore, the engine must only balance the frictional force.

Frictional force

On a horizontal road, $N = mg$

So friction is $f = \mu N = \mu mg$

Substituting values, $f = 0.04 \times 500 \times 9.8$ $f = 196\,\text{N}$

Work done by engine

At constant speed, engine force equals friction force, so work done over distance $s$ is $W = fs$

$W = 196 \times 4000$ $W = 784000\,\text{J}$

Convert into kJ

$784000\,\text{J} = 784\,\text{kJ}$

Final answer

$\boxed{784}$

Comparison with stored correct answer

Stored correct answer = $784$

This matches the derived result.