## Subtitles section Play video

• FOSSUM: Hi. My name's Mike Fossum.

• I'm the commander of Expedition 29.

• Welcome to the International Space Station.

• You may wonder why the International Space Station

• doesn't tumble in its orbit around the Earth.

• Well, we have a system that maintains our attitude --

• our attitude control system, if you will.

• There's really two ways to do this, to approach it.

• The first way would be to use rockets or small jets

• that would keep thrusting and pushing

• to nudge the Space Station into its correct attitude.

• Problem with using thrusters like that

• is it requires a lot of fuel

• that we have to continually ship up from the Earth.

• So, instead, we use angular momentum and gyroscopes.

• Let me demonstrate a little bit.

• Before we get into this,

• we really have to talk about the definitions of the concepts.

• It's really the conservation of angular momentum

• that's really important here.

• I have this flashlight rotating.

• In physics, this angular momentum of an object

• rotating about some reference point

• right here in the middle --

• that's a measure of the extent

• to which the object will continue to rotate

• around that point,

• unless it's acted on by an external torque.

• Now, torque has to do with the ability of a force

• to rotate an object

• and how far away from the center that force is applied.

• The angular momentum of a single, rotating body

• is equal to the product of its rotational inertia --

• that's just physical properties of the mass distribution,

• the rotational inertia --

• and angular speed, how fast is it moving.

• The angular momentum and rotational inertia --

• that includes the mass -- are constants in the system.

• Angular momentum of the system remains constant

• if no external force is acting on the system.

• This is the law of conservation of angular momentum.

• I can demonstrate this to you

• with me actually doing a little bit of a demonstration here,

• so I want you to watch this.

• I'm gonna have to unclip the microphone,

• or I'll tie myself up in knots.

• Again, what's going on as my body spins doing this

• and moving my arms in and out from being close to the body

• is when I bring the arms in closer to the body,

• the body's rotational inertia is decreased.

• The angular momentum must be conserved.

• As the rotational inertia is decreased,

• the angular speed increases and vice versa.

• This is called an inverse relationship.

• Now, no external force acts on the system,

• and thus the angular momentum about the axis

• must remain the same.

FOSSUM: Hi. My name's Mike Fossum.

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# Angular Momentum

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Wayne Lin posted on 2015/02/15
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