Solving Momentum Issues: 98kg Man & 86g Stone Speed Calculation

[AlKaiser]

1. The problem statement:
A 98 kg man lying on a surface of negligible friction shoves a 86 g stone away from himself, giving it a speed of 4.5 m/s. What speed does the man acquire as a result?



2. Homework Equations :
Pi = Pf
M1V1 + M2V2 = M1V1 + M2V2



3. The Attempt at a Solution :

I've tried the formula above, and I got this far:

98 x 0 + 0.086 x 0 = 98 x X + 0.086 x 4.5
0 = 98X + 0.387
-0.387 = 98X
-0.387/98 = X
X = -0.00394897959 m/s


That's what I got. I have a feeling that I've done something wrong. The online Assignment system is telling me my answer is wrong, so what am I doing wrong? Have I missed something? All help is greatly appreciated :D

 

[The Liberator]

…M1V1 + M2V2 = M1V1 + M2V2…

When you are substituting the velocity into the equation, the left side of the equation. The equation normally should look more like this:

$$m_a*u_a-m_b*u_b=m_a*v_a+m_b*v_b$$

By the way, I state "u" as the initial velocity of the object(s), and "v" as the final.

 

[sArGe99]

If you're taking the direction of the stone's velocity to be positive, the other has to be negative since its in the opposite direction. ;)