Momentum Conservation in a Swapping Chair Scenario in a Spacecraft

In summary, the spacecraft does move while the people swap seats, but it is stationary once they sit down again.
  • #1
nugget
47
0
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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  • #2
Welcome to PF!

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:
 
  • #3


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.
 
  • #4


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...
 

What is momentum in a spacecraft?

Momentum is the measure of the amount of motion a spacecraft has. It is calculated by multiplying the mass of the spacecraft by its velocity.

Why is momentum important in space travel?

Momentum is important in space travel because it is necessary for a spacecraft to change its direction or speed. Without momentum, a spacecraft would continue moving in a straight line at a constant speed.

How does momentum affect the trajectory of a spacecraft?

Momentum affects the trajectory of a spacecraft by determining how much force is needed to change its direction or speed. The greater the momentum, the more force is needed to change the spacecraft's trajectory.

How can momentum be conserved in a spacecraft?

Momentum can be conserved in a spacecraft by following the laws of physics, specifically Newton's third law of motion. This means that for every action, there is an equal and opposite reaction. In a spacecraft, this can be achieved by using thrusters in opposite directions to cancel out each other's momentum.

What factors can affect the momentum of a spacecraft?

The mass and velocity of a spacecraft are the main factors that affect its momentum. Other factors that can affect momentum include external forces, such as gravity or collisions with other objects in space.

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