How do satellites stay in orbit? Free fall or centripetal?

[DaveC426913]

Easy. Geo satellites are in orbit traveling at about 6900 MPH with an altitude of about 22,000miles. The exact same mechanism holds for satellites in low or high Earth orbit. It just so happens this altitude and speed produces an orbital period that matches Earth's period of rotation. To us the geo looks like it is motionless but it is definitely moving very fast.

I think you misunderstand. Reread. D_H is clever enough to understand how geostat sats work.


His comment was in the context of an Earth-based frame of reference, where the motion of the satellite is zero. It is a valid FoR, but one must create a force to explain what holds these stationary objects up.

Something I'm curious about geosats. Are the antennas on them directional? If so the satellite would have to rotate 360 degrees on its axis during every orbit to keep the antennas facing the earth. Just like the moon keeps the same side to Earth by rotating during the journey.

I'm pretty sure most satellites are spun to give them stability. I guess gyroscopically, that would mean they do not rotate (i.e. their rotational axis points to the same position on the celestial sphere permanently).

 

[ruko]

I think you misunderstand. Reread. D_H is clever enough to understand how geostat sats work.


His comment was in the context of an Earth-based frame of reference, where the motion of the satellite is zero. It is a valid FoR, but one must create a force to explain what holds these stationary objects up.I'm pretty sure most satellites are spun to give them stability. I guess gyroscopically, that would mean they do not rotate (i.e. their rotational axis points to the same position on the celestial sphere permanently).

Geo sats only appear stationary or motionless to us and that's because we are moving right along with them. They have to be speeding along at 6900 MPH to remain in the geo orbit otherwise they will fall down. This is very basic stuff. We don't have to create a force to explain what holds these "geo stationary" objects up. It is the same force that holds any satellite up. YouTube has a few good animations of orbiting geo sats.

 

[DaveC426913]

Geo sats only appear stationary or motionless to us and that's because we are moving right along with them. They have to be speeding along at 6900 MPH to remain in the geo orbit otherwise they will fall down. This is very basic stuff. We don't have to create a force to explain what holds these "geo stationary" objects up. It is the same force that holds any satellite up. YouTube has a few good animations of orbiting geo sats.

Again, you are misunderstanding.

Everyone here knows how geostat sats work in an inertial frame of reference. The discussion is about the non-inertial rotating FoR.

 

[ruko]

Again, you are misunderstanding.

Everyone here knows how geostat sats work in an inertial frame of reference. The discussion is about the non-inertial rotating FoR.

Sorry I guess I only think from a practical position. When someone asks how do satellites stay up, I try to answer from a practical standpoint. "non-inertial rotating FoR" Can you explain to me how this relates to geo sats?

 

[Naty1]

centripetal force: F = ma =mv²/r and v = wr so F = mw² r...

The gravitational force on either body must equal the centripetal force needed to keep each moving in their own circular orbit...GMm/(R+r)² = mw²r
where M is the mass of Earth and R it's orbital radius about the center of mass for the two systems, m,r for the satellite.
Since the orbital radius opf the Earth (R) is insignificant (assume R = 0) the above simplifies.

so I like centripetal.

 

[D H]

Sorry I guess I only think from a practical position. When someone asks how do satellites stay up, I try to answer from a practical standpoint. "non-inertial rotating FoR" Can you explain to me how this relates to geo sats?

ruko, you were the one who invoked the concept of centrifugal force to explain orbits in post #26. If you don't know the physics of rotating frames of reference, why did you do that?

 

[brainstorm]

Inertia plus the gravity well of the Earth permit satellites to orbit. Centrifugal force is in fact, inertia.

Maybe not my place to defend here, but I went back and looked at this post because it was cited. It seems to be a very good, clear, and simple post to me because he seems to basically be saying that an object in a curved trajectory has inertia which causes it to tend outward if released from the constraints that keep it curving.

He seems to be saying that if there wasn't inertia, the object would fall; and if there wasn't a gravity well, the object would proceed in a straight line instead of orbiting.

Plus, isn't centrifugal force always just inertia? Gravity is centripetal force because it pulls objects toward the center of the gravity well, against their inertia and any escape propulsion.

 

[ruko]

Maybe not my place to defend here, but I went back and looked at this post because it was cited. It seems to be a very good, clear, and simple post to me because he seems to basically be saying that an object in a curved trajectory has inertia which causes it to tend outward if released from the constraints that keep it curving.

He seems to be saying that if there wasn't inertia, the object would fall; and if there wasn't a gravity well, the object would proceed in a straight line instead of orbiting.

Plus, isn't centrifugal force always just inertia? Gravity is centripetal force because it pulls objects toward the center of the gravity well, against their inertia and any escape propulsion.

Thank you brainstorm.

DH:
I did not invoke centrifugal force. Centrifugal force is a fictitious force. The last time I checked fictitious means fake or does not exist unless a PHD has come up with a new definition for the word. What the average person calls centrifugal force is really the effects of inertia, like the ball rolling to the door and hugging the door during a turn (My Post 32) or the water not falling out of the pail when swung overhead. You are correct, I don't understand rotating frames of reference but even if I did, how would that help me understand geosats? I know how they get there, I know what keeps them there and I know what they are used for. Do I really need any more info?

 

[DaveC426913]

I don't understand rotating frames of reference but even if I did, how would that help me understand geosats? I know how they get there, I know what keeps them there and I know what they are used for. Do I really need any more info?

Hurricanes on Earth are understood as a result of the Coriolis Force. While the Coriolis Force is fictitious just like centrifugal force, it makes more sense to understand weather in our rotating frame of reference using this fictitious force than it does to examine weather from an inertial frame of reference.

I'm sure there is an equivalent for geosats.

 

[danielandpenn]

Hmmmm. Free fall is the name of the situation the satellite is in, but its not what keeps it orbiting. The centripetal acceleration causes the object to constantly accelerate toward the center of the Earth and thus orbit. Free fall is a result of this acceleration and the objects velocity going around the earth, not the cause of the orbit itself...That help?

What is causing the centripetal acceleration? I'm new to physics. Thanks!

 

[jtbell]

What is causing the centripetal acceleration?

The centripetal force, which in this case is gravity.

 

[rcgldr]

The answer choices were inertia, free fall, and centripetal acceleration.

The correct answer should be all of the above. The satellite's inertia, combined with it's speed perpendicular to gravity, and it's altitude at any point in time, determine the orbital path. The satellite is technically in free fall even in a circular orbit, and centripetal force, in this case gravity, is what cause the satellite to curve to maintain the orbital path. If you also consider the more generic case of an elliptical orbit, then a component force of gravity is in the direcion of travel (except at the asymtopes of the ellipse), so a "free fall" component, and also has a component of force perpendicular to the direction of travel, (centrpetal force).

 

[brainstorm]

The correct answer should be all of the above. The satellite's inertia, combined with it's speed perpendicular to gravity, and it's altitude at any point in time, determine the orbital path. The satellite is technically in free fall even in a circular orbit, and centripetal force, in this case gravity, is what cause the satellite to curve to maintain the orbital path. If you also consider the more generic case of an elliptical orbit, then a component force of gravity is in the direcion of travel (except at the asymtopes of the ellipse), so a "free fall" component, and also has a component of force perpendicular to the direction of travel, (centrpetal force).

Can any straight-line trajectory ever be possible except for an object moving directly toward the center of a gravitational field? Aren't all other trajectories relative to some gravitational fulcrum, however slight or distant the source of gravity?

 

[jimmyct]

a explanation of the moon staying in orbit
http://math.ucr.edu/home/baez/physics/General/Centrifugal/centri.html

 

[astronomer121]

this may help u out http://gravity-nirmal.blogspot.com/2010/04/Earth's-satellites.html

 

[brainstorm]

How is a state of rest to be defined within a gravitational field?

Satellites are always at rest relative to themselves and are only in motion relative to other satellites and/or the fulcrum of orbit.

If they are in a circular orbit, how is their motion relative to the fulcrum to be described if the fulcrum is conceived as a point without a surface?

The simple answer is that a certain amount of energy is required to counteract the attractive force of gravity. This energy is expressed perpendicular to the line between the satellite and the orbital fulcrum.

When the energy of orbit is balanced against the force of gravity, constant distance is sustained between satellite and fulcrum. When it becomes unbalanced, the distance between satellite and fulcrum increases or decreases, along with the momentum of the satellite.

But how can momentum be described as changing in the context of the satellite's relationship with itself? It may accelerate or decelerate relative to a third object other than itself and the fulcrum, but without that third object, it can only move closer or farther from the fulcrum, with its energy being expressed as resistance to gravitational pull, correct?