(Un) Realistic Situation 1

Classical Mechanics Level 2

Two fat astronauts each of mass 120 k g 120 kg are traveling in a closed spaceship moving at a speed of 15 k m s − 1 15 km s^{-1} in the outer space far away from any other object. The total mass of the spaceship including the astronauts is 660 k g 660 kg .

If the astronauts try to do slimming exercise & succeed in bringing their weights down to 90 k g 90 kg each (this is sure to make the women jealous), with what velocity will the spaceship finally move in k m s − 1 km s^{-1} ?

You can also try (Un) Realistic Situation #2 .


The answer is 15.

Ameya Salankar by Ameya Salankar , 23, India

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3 solutions

Raghav Chaudhary
May 18, 2014

The first thing that I thought was conservation of momentum. But then I realized that the two astronauts were in a closed spaceship, i.e. they were sort of "internal" to the spaceship. Since conservation of momentum does not apply for internal forces (or in this case, internal change in mass), the velocity of the spaceship will not change.

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Cheah Chung Yin
May 27, 2014

the spaceship in constant velocity is equivalent to an object at rest. When an object at rest changes its mass, it is still at rest.

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Mirtaki Tajwar
May 15, 2014

this can be solved by the conservation of energy, mainly the kinetic energy, after finding the mass of the spaceship, we can solve it by the conservation law!!!!!!!!!!!!

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