Momentum Issues

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

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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. ;)

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