#### nugget

1. Homework Statement
John (90kg) and Julie (60kg) are in a spacecraft (50kg). They swap chairs, which are 4m apart, located equal distances from the Centre of Mass of the spacecraft.

Questions: Why does the spacecraft NOT move after they take their seats?
How far does the spacecraft move? Which way?

All observations are in the frame in which the spacecraft was initially stationary.

2. Homework Equations

I don't know which equations to use, the question also says that they hope we attempt to answer this question using momentum principles as well as the centre of mass approach.

3. The Attempt at a Solution

The only thing i have is that the CM should initially be 1.6 meters from John.

This question was especially confusing as they ask why the spacecraft doesn't move, and then where it moves to... It seems to make no sense, all i can think of is this: The centre of mass moves when the people swap seats. It's always going to be closer to John than to Julie in the spacecraft because he is heavier. If we use the CM as our frame of reference, the spacecraft will appear to move in the opposite direction that the centre of mass does while the CM will not appear to move at all.

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#### tiny-tim

Homework Helper
Welcome to PF!

Hi nugget! Welcome to PF!
John (90kg) and Julie (60kg) are in a spacecraft (50kg). They swap chairs, which are 4m apart, located equal distances from the Centre of Mass of the spacecraft.

Questions: Why does the spacecraft NOT move after they take their seats?
How far does the spacecraft move? Which way?

This question was especially confusing as they ask why the spacecraft doesn't move, and then where it moves to... It seems to make no sense …
Yes, it's a really confusing question …

I had to read it three times before I saw what it was getting at

it's saying that the spacecraft does move while they change seats, but is stationary once they sit down again

#### HallsofIvy

Homework Helper

Actually, I find the reference to a "spacecraft" confusing since I started imagining it in orbit. Think, instead, of two people sitting on a sled on ice. The center of mass of the system must stay stationary since there is no external force. Calculate the center of mass both before and after they change seats. While they are changing seats the sled moves so that the center of the mass moves from one place to the other.

#### nugget

thanks guys, that's helped the question make a lot more sense :)

Still don't know quite how to answer the question using momentum principles...

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