The momentum, p, of any object having mass m and the velocity v is
Let and
be the masses of the large truck and the car respectively, and
and V_S be the velocities of the large truck and the car respectively.
So, by using equation (i),
the momentum of the large truck
and the momentum of the small car .
If the large truck has the same momentum as a small car, then the condition is
The equation (ii) can be rearranged as
So, the first scenario:
So, to have the same momentum, the ratio of mass of truck to the mass of the car must be equal to the ratio of velocity of the car to the velocity of the truck.
The other scenario:
So, to have the same momentum, the ratio of mass of truck to the velocity of the car must be equal to the ratio of mass of the car to the velocity of the truck.