Once you can compute the momentum of an object, finding the momentum of a system is easy. A system is just a group of objects, and to find the total momentum is just a matter of adding.

The equation may look fancy.

pt = Σ pi

But all that means is that you find the total momentum by adding up all the individual momentums.

Example:
Three students are running in a field. One is 40 kg running east at 5 m/s, one is 50 kg running east at 7m/s and the other is 45 kg running west at 6 m/s. What is the total momentum of the system?

Step 1: find each momentum

Student 1 Student 2 Student 3

m = 40 kg
v = 5 m/s east = +5 m/s
p = mv = (40)(5)

p = +200 kg m/s

m = 50 kg
v = 7 m/s east = +7 m/s
p = mv = (50)(7)

p = +350 kg m/s

m = 45 kg
v = 6 m/s west = -6 m/s
p = mv = (45)(-6)

p = - 270 kg m/s

Step 2: add them up

pt = 200 + 350 + (-270) = 280 kg m/s