How Do Spacecraft Orbit Earth? Angular Momentum Explained By NASA
How is it possible for the ISS to stay in orbit? Learn more about the science behind orbiting Earth and more in this NASA "STEMonstrations" video.
Credit: NASA Johnson Space Center
Credit: NASA Johnson Space Center
Transcript
00:00 [ Music ]
00:16 >> Hello, my name is Sultan Al-Niyadi
00:17 and I'm an astronaut living and working
00:19 on board the International Space Station.
00:22 Any idea how it's possible for the space station
00:24 to continuously orbit Earth 250 miles above the surface?
00:28 And why at 17,500 miles per hour?
00:32 What would happen if the station sped up or slowed down?
00:36 We are going to explore those questions and more
00:38 by investigating the connection between the angular momentum
00:42 and the orbits in our microgravity environment.
00:45 But first, you need to know a couple of other terms.
00:49 Let's get started.
00:51 Before we dive into centripetal force, it's important to look
00:54 at Newton's first law of motion,
00:56 which states that an object will continue moving
00:59 with a constant velocity along a straight path unless acted upon
01:03 by a net external force.
01:05 This means that the space station will move along a
01:07 straight path if it weren't
01:09 for one key external force acting on it,
01:11 Earth's gravitational pull.
01:14 Another name for this external force is centripetal force.
01:18 A centripetal force is any net force
01:20 that keeps an object moving along a circular path.
01:23 Gravity, in this case, is a centripetal force
01:26 because it is the force that is keeping our space station moving
01:29 in its circular path around Earth.
01:32 [ Music ]
01:36 Okay, now you know that gravity constantly pulls a moving
01:39 object with linear momentum inward just enough to cause it
01:42 to travel in a curved path, making its momentum angular.
01:46 The International Space Station maintains this balance
01:51 between gravity and linear momentum by traveling
01:54 at the required 17,500 miles per hour
01:57 to maintain an altitude of 250 miles.
02:01 This is considered low Earth orbit.
02:03 It is high enough to encounter very little interference
02:05 from the atmosphere, but low enough
02:07 to be relatively easy to travel to.
02:10 Let me show you some examples
02:11 of angular momentum being conserved
02:13 in the microgravity environment aboard the station.
02:16 I will apply a force to set this yo-yo in motion.
02:19 The force of tension is transferred through the string,
02:22 which is a centripetal force keeping this yo-yo revolving
02:25 around my hand.
02:26 But what happens when I let go of the string?
02:28 Once the tension from the string is removed, the object continues
02:31 to follow Newton's first law of motion.
02:34 It keeps moving at a constant velocity along a straight path
02:37 relative to the space station.
02:39 Now, what happens to the motion of the yo-yo
02:41 if we increase the centripetal force
02:43 by increasing the tension in the string?
02:45 As I'm holding the string between two fingers on one hand
02:49 to keep the axis of the rotation stable,
02:51 I'm going to pull the string with my other hand,
02:54 increasing the tension and centripetal force
02:56 and decreasing the radius of the yo-yo's orbit.
02:59 As the radius of the yo-yo's orbit decreases,
03:01 its velocity increases.
03:03 Angular momentum is a product of an object's velocity, mass,
03:07 and the radius of its orbit from an object's center.
03:11 If you only have centripetal force,
03:13 angular momentum must also be conserved.
03:15 So if the radius of its orbit decreases,
03:18 its velocity must increase in order
03:20 to maintain its angular momentum.
03:23 Let's try this again, but this time,
03:25 I'll decrease the tension on the string,
03:28 lowering the centripetal force and increasing
03:30 the radius of the yo-yo's orbit.
03:34 If you thought the velocity of the yo-yo would decrease,
03:37 you were right.
03:38 Since angular momentum must be conserved,
03:40 if the radius of an orbit is increased,
03:43 the velocity of the yo-yo must decrease.
03:48 As you can see, there is an inverse relationship
03:50 between the radius of the orbit and the yo-yo's velocity.
03:53 I was able to change the velocity of the yo-yo
03:56 by increasing and decreasing the centripetal force
03:58 in the system.
03:59 We can't do this with the orbit of the station
04:02 or other satellites, because we can't change the pull
04:04 of gravity exerted by Earth.
04:06 Instead, to keep the station in a stable, circular orbit,
04:10 we used thrusters that can help maintain the constant speed
04:13 of 17,500 miles per hour.
04:18 To learn more about these topics,
04:19 check out the corresponding classroom connection
04:21 to conduct your own experiment and discover
04:23 other ways angular momentum plays a part in your daily life.
04:27 Thank you for exploring some physics with me today,
04:29 and see you soon.
04:31 [MUSIC PLAYING]
04:35 (dramatic music)