The work needed to halt a car equals its initial kinetic energy, calculated as:
\[
K.E. = \frac{1}{2} m v^2
\]
Where:
- \( m = 2000 \, \text{kg} \)
- \( v = 18 \, \text{km/h} = 5 \, \text{m/s} \) (converted from km/h to m/s)
Using these values:
\[
K.E. = \frac{1}{2} \times 2000 \times 5^2 = 1000 \times 25 = 25000 \, \text{joules} = 2.5 \times 10^5 \, \text{joules}
\]
Therefore, the work required to stop the car is \( 2.5 \times 10^5 \, \text{joules} \).