Given data:
Mass of the car (m) = 1200 kg
Initial velocity (u) = 90 km/hr = 25 m/s
Final velocity (v) = 18 km/hr = 5 m/s
Time taken (t) = 4 s

Calculating acceleration:
Acceleration (a) can be calculated using the equation:
a = (v - u) / t

Substituting the given values:
a = (5 - 25) / 4
a = -20 / 4
a = -5 m/s²

Hence, the acceleration of the car is -5 m/s².

Calculating change in momentum:
Change in momentum (Δp) can be calculated using the equation:
Δp = m * (v - u)

Substituting the given values:
Δp = 1200 * (5 - 25)
Δp = 1200 * (-20)
Δp = -24000 kg·m/s

The change in momentum is -24000 kg·m/s.

Calculating the magnitude of the force:
To calculate the magnitude of the force required, we can use Newton's second law of motion:
F = m * a

Substituting the mass and acceleration values:
F = 1200 * (-5)
F = -6000 N

The magnitude of the force required is 6000 N.

Explanation:
The given problem involves a car of mass 1200 kg that is initially moving with a uniform velocity of 90 km/hr (25 m/s). The car is then slowed down to a velocity of 18 km/hr (5 m/s) in a time of 4 seconds.

To calculate the acceleration, we use the formula a = (v - u) / t, where v is the final velocity, u is the initial velocity, and t is the time taken. Substituting the given values, we find that the acceleration of the car is -5 m/s². The negative sign indicates that the car is decelerating.

Next, we calculate the change in momentum using the formula Δp = m * (v - u), where Δp is the change in momentum, m is the mass of the car, v is the final velocity, and u is the initial velocity. Substituting the given values, we find that the change in momentum is -24000 kg·m/s. The negative sign indicates that the momentum of the car decreases.

Finally, we calculate the magnitude of the force required using Newton's second law of motion, F = m * a, where F is the force, m is the mass of the car, and a is the acceleration. Substituting the mass and acceleration values, we find that the magnitude of the force required is 6000 N. The negative sign indicates that the force is acting in the opposite direction to the motion of the car, causing it to slow down.

Acceleration= -5m/s^2Change in momentum=-24000kgm/sForce=6000 N.