Momentum Conservation in a Swapping Chair Scenario in a Spacecraft

AI Thread Summary
In the swapping chair scenario within a spacecraft, the spacecraft does not move after John and Julie take their seats due to the conservation of momentum and the center of mass remaining stationary. While they are changing seats, the spacecraft does shift slightly to maintain the overall center of mass, which remains constant since no external forces act on the system. The heavier individual, John, causes the center of mass to be closer to him, influencing the movement of the spacecraft during the swap. Once they are seated, the spacecraft stabilizes, appearing stationary in the initial frame of reference. Understanding this concept clarifies the confusion around the spacecraft's movement during the seat exchange.
nugget
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Momentum in a spacecraft , please help!

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.

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.

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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Hi nugget! Welcome to PF! :smile:
nugget said:
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 :frown: …

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


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.
 


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...
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

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Thread 'Struggling to understand the concept of this question (ice cube in water optics question)'

TL;DR Summary: I came across this question from a Sri Lankan A-level textbook. Question - An ice cube with a length of 10 cm is immersed in water at 0 °C. An observer observes the ice cube from the water, and it seems to be 7.75 cm long. If the refractive index of water is 4/3, find the height of the ice cube immersed in the water. I could not understand how the apparent height of the ice cube in the water depends on the height of the ice cube immersed in the water. Does anyone have an...
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