Given data:
* The mass of the van is 1800 kg.
* The initial velocity of the van is 9m/s.
Solution:
The initial momentum of the van in the terms of the mass and the initial velocity is,

where m is the mass of the van and u is the initial velocity,
Substituting the known values,

Thus, the initial momentum of the van is 16200 kgm/s.
(b). The final velocity of the van is zero.
Then, the force acting to stop the van in 5 seconds is,

Thus, the magnitude of the force acting to stop the van is 3240 N.