# Solve for mass of a second object in a momentum question

1. Nov 19, 2015

### AtomicBoom

1. The problem statement, all variables and given/known data
Two objects collide head-on, and are stuck together after collision. Find mass of second object.
Givens:
m1 = 120 kg
v1 = 12 m/s(N)
m2 = ?
v2 =18 m/s (S)
v' = 4.0 m/s (S)

2. Relevant equations
m1v1 + m2v2 = m1v1' + m2v2'
v'= (m1v1 + m2v2) / (m1+m2)

3. The attempt at a solution
-inelastic collision
-combined mass after collision
-therefore, final velocity will be the same
-I'm not sure on how to rearrange the equation so only one m2 is present in the equation

v'= (m1v1 + m2v2) / (m1+m2)
(4) = ((120)(12) + m2(18)) / (120 + m2)
4(120 + m2) = 1440 + m2(18)
480 + 4m2 = 1440 + m2(18)
480 - 1440 = 18m2 - 4m2
-960 = 14m2
m2 = 68.6 kg

The actual answer is supposed to be 140 kg.

(sorry if this looks confusing, I'm new to this website and haven't quite figured it out yet.)
Thanks

2. Nov 19, 2015

### haruspex

You need to be careful with signs. Each velocity quoted has a stated direction, N or S. Pick whether N or S should be the positive direction and express the velocities accordingly.

3. Nov 20, 2015

### AtomicBoom

Oh right, okay.
When I make N positive and S negative and solve it, I get around the right answer.

Thank You!