Question

In an isolated system, bicycle 1 and bicycle 2, each with a mass of 15 kg, collide. bicycle 1 was moving to the right at 1 m/s, while bicycle 2 was moving to the left at 3 m/s. what is the magnitude of their combined momentum before the crash?

Answer 1

To do this, all you need to do is get the momentum of each and get the sum. The formula of momentum is:

p=mv

where:
p = momentum
m = mass
v = velocity

Bicycle 1 and 2 have the same mass of 15 kg

Bicycle 1 v = 1 m/s
Bicycle 2 v = 3 m/s but because it was coming from the opposite direction then we get a negative velocity. The negative just indicates the direction so it will be -3m/s

m1v1+m2v2

=(15kg)(1m/s) + (15kg)(-3m/s) 
= 15kg.m/s + (-45kg.m/s)
=-30kg.m/s

Their combined momentum before the crash is 30kg.m/s. We take out the negative sign because the question is asking only for the magnitude and not the direction and remember the negative sign indicates the direction.