Final answer:
The deceleration of a car coming to a stop from 18 m/s cannot be calculated without additional information such as stopping time or distance. The kinematic equation required is a = (v^2 - u^2) / (2s).
Step-by-step explanation:
To calculate the deceleration of a 1200 kg car coming to a complete stop from a speed of 18 m/s, we use the formula for average acceleration: a = Δv/Δt, where Δv is the change in velocity and Δt is the change in time. Since we are not given the time taken to stop the car, we can use the kinematic equation v^2 = u^2 + 2as, where v is the final velocity, u is the initial velocity, a is acceleration, and s is the displacement. In this case, the car comes to a complete stop, so the final velocity v is 0 m/s, and the initial velocity u is 18 m/s. Assuming there is no information about the displacement (distance taken to stop), we cannot provide a specific numerical answer. However, deceleration can be calculated if the stopping distance was provided using the rearranged kinematic equation: a = (v^2 - u^2) / (2s).
In this situation, you need additional information, such as the time taken to stop or the distance over which the car stopped, to calculate the deceleration (negative acceleration).