(a) Calculation of acceleration

Here, Initial velocity,u=90km/h

`=(90xx1000m)/(60xx60s)`

=25 m/s

Final velocity,v=18 km/h ltbr? `=(18xx1000m)/(60xx60s)`

=5m/s

Acceleration,a=? (To be calculated)

And, Time,t=4s

Now, v=a+ut

`5=25+axx4`

4a=5-25

4a=-20

`a=-20/4`

`a=-5m//s^(2)`

Thus, the acceleration of the motorcar is, `-5 m//s^(2)`. It is actually negative acceleration or retardation.

(b) Calculation of change in momentum

Change in momentum =mv-mu

`=1200xx5-1200xx25`

`=6000-30000`

`=-24000kg.m//s`

Thus, the change in momentum is 24000 kg. m/s

(c) Calculation of magnitude of force

Force, F=`mxxa`

`F=1200xx(-5)`

F=-6000N

Thus, the magnitude of force required is 6000 newtons. The negative sign shows that this force is opposing the motion.