Here, `m=1200kg, u=90km//h=(90xx1000m)/(60xx60)=25m//s`

`v=18km//h=(18xx1000m)/(60xx60s)=5m//s, t=4s`

Acceleration, `a=(v-u)/t=(5-25)/4= -5m//s^(2)`

change in momentum `= m(v-u)=1200(5-25)= -24000kg m//s`

Change in momentum `= m(v-u)=1200(5-25)= -24000kg m//s`

Force required, `F=ma=1200(-5)= -6000N`

Magnitude of force required `=6000N`

Negative sign shows that force is opposing the motion.