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## Homework Statement

A 1000 kg horse trailer with frictionless wheels is sitting in a level parking lot. The trailer is 4 m long, and its center of mass is at its center. Its passenger, a 500 kg horse, breaks free from its stall at one end of the trailer and walks to the other end. How far does the trailer move relative to the ground? Treat the horse as a point particle. The mass of the trailer above does not include the 500 kg horse.

## Homework Equations

[itex]x_{cm} = \frac{1}{m}\sum_{i=0}^{n} m_ix_i[/itex]

## The Attempt at a Solution

I knew that x

_{cm}*m = (1000 kg)(2 m) + (500 kg)(0 m), which is 2000 kg*m, therefore the new position must be (2000 kg*m) = (1000 kg)x + (500 kg)(4 m) because no external forces are acting upon it. This yields x = 0m, but the correct answer is x = 4/3 m. Why is that?