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
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00:00Hello, my name is Sultan Alniadi and I'm an astronaut living and working aboard the International
00:20Space Station.
00:22Any idea how it's possible for the space station to continuously orbit Earth 250 miles above
00:27the surface?
00:29And why at 17,500 miles per hour?
00:33What would happen if the station sped up or slowed down?
00:35We are going to explore those questions and more by investigating the connection between
00:40the angular momentum and the orbits in our microgravity environment.
00:45But first, you need to know a couple of other terms.
00:49Let's get started.
00:52Before we dive into centripetal force, it's important to look at Newton's first law of
00:56motion, which states that an object will continue moving with a constant velocity along
01:00a straight path unless acted upon by a net external force.
01:05This means that the space station will move along a straight path if it weren't for one
01:09key external force acting on it, Earth's gravitational pull.
01:14Another name for this external force is centripetal force.
01:18A centripetal force is any net force that keeps an object moving along a circular path.
01:24Gravity, in this case, is a centripetal force because it is the force that is keeping our
01:28space station moving in its circular path around Earth.
01:32Okay, now you know that gravity constantly pulls a moving object with linear momentum
01:41inward just enough to cause it to travel in a curved path, making its momentum angular.
01:49The International Space Station maintains this balance between gravity and linear momentum
01:53by traveling at the required 17,500 miles per hour to maintain an altitude of 250 miles.
02:01This is considered low Earth orbit.
02:03It is high enough to encounter very little interference from the atmosphere but low enough
02:07to be relatively easy to travel to.
02:10Let me show you some examples of angular momentum being conserved in the microgravity environment
02:15aboard the station.
02:16I will apply a force to set this yoyo in motion.
02:19The force of tension is transferred through the string, which is a centripetal force keeping
02:24this yoyo revolving around my hand.
02:26But what happens when I let go of the string?
02:28Once the tension from the string is removed, the object continues to follow Newton's first
02:33law of motion.
02:34It keeps moving at a constant velocity along a straight path relative to the space station.
02:40Now what happens to the motion of the yoyo if we increase the centripetal force by increasing
02:44the tension in the string?
02:46As I'm holding the string between two fingers on one hand to keep the axis of the rotation
02:50stable, I'm going to pull the string with my other hand, increasing the tension and
02:55centripetal force and decreasing the radius of the yoyo's orbit.
02:59As the radius of the yoyo's orbit decreases, its velocity increases.
03:04Angular momentum is the product of an object's velocity, mass, and the radius of its orbit
03:09from an object's center.
03:11If you only have centripetal force, angular momentum must also be conserved.
03:15So if the radius of its orbit decreases, its velocity must increase in order to maintain
03:20its angular momentum.
03:23Let's try this again, but this time I'll decrease the tension on the string, lowering the centripetal
03:29force and increasing the radius of the yoyo's orbit.
03:34If you thought the velocity of the yoyo would decrease, you were right.
03:38Since angular momentum must be conserved, if the radius of an orbit is increased, the
03:43velocity of the yoyo must decrease.
03:48As you can see, there is an inverse relationship between the radius of the orbit and the yoyo's
03:53velocity.
03:54I was able to change the velocity of the yoyo by increasing and decreasing the centripetal
03:58force in the system.
03:59We can't do this with the orbit of the station or other satellites because we can't change
04:04the pull of gravity exerted by Earth.
04:06Instead, to keep the station in a stable circular orbit, we used thrusters that can help maintain
04:12the constant speed of 17,500 mph.
04:18To learn more about these topics, check out the corresponding Classroom Connection to
04:21conduct your own experiment and discover other ways angular momentum plays a part in your
04:26daily life.
04:27Thank you for exploring some physics with me today, and see you soon.