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**P**

_{initial}=

**P**

_{final}.

So I do that for a mass M

_{1}colliding with another mass M

_{2}that’s initially at rest,

M

_{1}x

**V**

_{1}

_{initial}+ 0 = M

_{1}x

**V**

_{1}

_{final}+ M

_{2}x

**V**

_{2}

_{final}

and I get an equation for the velocity

**V**

_{2}

_{final},

**V**

_{2}

_{final}= (M

_{1}/M

_{2}) x (

**V**

_{1}

_{initial}-

**V**

_{1}

_{final}) .

Now let’s say that M

_{2}is so large that in the limit it becomes infinite. In that case,

**V**

_{2}

_{final}= 0, so that

**V**

_{1}

_{final}=

**V**

_{1}

_{initial}.

But since M

_{1}bounces off of M

_{2}, shouldn't

**V**

_{1}

_{final}and

**V**

_{1}

_{initial}be pointing in opposite directions?