Momentum Relative to Surface / Plug-and-Chug

[Lank]

Homework Statement

Stevie stands on a rolling platform designed for moving heavy objects. The platform has mass of 76 kg and is on a flat floor, supported by rolling wheels that can be considered to be frictionless. Stevie's mass is 43 kg. The platform and Stevie are stationary when she begins walking at a constant velocity of +1.2 m/s relative to the platform. (a) What is the platform's velocity relative to the floor? (b) What is Stevie's velocity relative to the floor?

Homework Equations

(p)i = (p)f Conservation of Momentum

The Attempt at a Solution

Used Conservation of Momentum, but I am having trouble establishing a reference point.

43x1.2 + 76xV2f = 0 - That's for the the platform and Stevie.

.521x119 = 43xV1f + 76xV2f - That's for the entire system...I think.

As stated above, I think my problem is finding the reference point. I would normally spend more time with this one by myself, but I have a quiz on this fairly soon.


The answers:
(a) −0.43 m/s
(b) 0.77 m/s

 

[BishopUser]

Your first equation would give you the correct answer if you considered that the platform has additional mass as a result of someone standing on it.

 

[Lank]

Of course it would.

You're the man BishopUser. Thanks dude.