What is the size and direction of the total combined momentum of the two trucks before they collide? Use conservation of momentum.

*The thing I dont understand is that I thought to get the size and direction of the total COMBINED momentum the two trucks have to collide first...otherwise how can i give 1 size and direction for two trucks...and combine it???

Calculate the momentum for each truck then add those two results. The resulting value is the total combined momentum. The direction of the final momentum is the direction of the resulting vector.

Suppose that the masses of the trucks are the same and the truck comming from the left has the greater speed. The truck comming from the left will have the greater momentum. The total momentum vector will then point to the right.

The important thing to keep in mind that the velocity is a vector quantity. If you're working in one dimension then you can work with the component of the momentum only. The velocity will then have a signed value according to the direction the object is moving. If it's moving to the right then the velocity will have a negative sign. If the object is moving to the right then it will have a positive sign.

Example: Let each truck have a mass of m = 10,000 kg. Let the speed of the truck coming from the left be 10 m/s and the speed of the truck comming from the right be 11 m/s. The velocity of the left truck is V_L = 10 m/s. The velocity of the right truck is V_R = -11 m/s.

The momentum of the left truck is

P_L = mV_L = (10,000 kg) (10 m/s) = 100,000 kg m/s

The momentum of the right truck is

P_R = mV_L = (10,000 kg) (-11 m/s) = -110,000 kg m/s

The total momentum is P = P_L + P_R or

100,000 kg m/s -110,000 kg = -10,000 kg m/s

The total momentum is to the left and has a magnitude of 10,000 kg m/s.