
66 Questioning the Universe: Concepts in Physics
P
2
= P
1
This time the initial momentum is not zero, but has the value of m
b
v
b
= 10*15 = 150
kg-m/s. We therefore write
m
1
v
1
+ m
2
v
2
= 6(20) + 4(−v
2
) = 150
or
−4v
2
= 150 − 120 = 30
or
v
2
= −7.5 m/s
What does the minus sign mean in the answer? It means that our assumption
as to the velocity direction of the 4 kg mass was incorrect, and that it too moves to
the right. Note that in Figure 8.2, we have chosen (assumed, guessed) that the 4 kg
mass went to the left, which means, according to our convention, that in the equation
above, we must assign its velocty as being negative (−v
2
). The minus sign i ...