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## AP®︎/College Physics 1

### Course: AP®︎/College Physics 1>Unit 4

Lesson 4: Newton's law of gravitation

# Space station speed in orbit

Speed necessary for the space station to stay in orbit. Created by Sal Khan.

## Want to join the conversation?

• How does the craft in space get up to that speed without the jets and how does it maintain that speed?
• The craft uses a propulsive engine a bit like a jet to escape earth's atmosphere and get to the right height and then it lets gravity accelerate it back to earth. But it uses its engines to give it enough momentum to miss earth and so fall constantly in an orbit. Its speed is maintained purely by gravity, maybe using it's engines every so often if its speed deviates too much.
• Where the gravitational force is more at poles or equator ?
• Wow! That's a fascinating question!

This is what I found. It was posted in 2011:

Just a few weeks ago the European Space Agency released results from its satellite GOCE which answers this question - In general it seems that "Gravitational acceleration at Earth’s surface is about 9.8 m/s², varying from a minimum of 9.788 m/s² at the equator to a maximum of 9.838 m/s² at the poles." from the same site. But there is a lot of regional variation - some of which we cannot explain.

Really interesting- thank you for asking!!
• How does one derive centripetal acceleration =v^2/r
• How would you calculate the speed for an elliptical orbit?
• Speed is always distance per unit of time, regardless of the path.
• If you were at the center of the earth (assuming that the heat and pressure don't obliterate you and there is a space for you to stand), what would happen? Would you stay there? Would you be ripped apart as gravity pulls you in each direction?
• At , How is a,c = v^2/r ? Is there a video to explain this concept?
• I was wondering, does a rocket actually need 11.2 km/sec of escape velocity initially to escape from the earth because that seems very crazy if so.
• No. 11.2 km/sec is needed to completely escape Earth and start orbiting the Sun. Hence the term "escape velocity." Its the velocity you need to escape Earth's gravity well for good completely and then get into a heliocentric orbit.

Real rockets don't go that fast, they just get around 27,000 kmph to get into orbit, which is just around 7.6 km/s.

That is the speed where you are falling fast enough where you miss the Earth, or in other words you acheive orbit at that rate.

For instance, if you want to go to Mars, what they do first is they get into orbit, 7.6 km/s and then from there on do another burn to get their velocity to 11.2 km/sec to escape Earth's orbit and get onto the transfer orbit towards the planet.
• If I throw a hammer on the surface of the moon since there is no atmosphere all the potential energy will be converted to heat energy, contrary to what happens on earth where a part of energy gets converted to sound as well?
• Hello Krishnaonlyspam,

What a fun question!

Let's talk about the sound part.

If we bring a bell to the moon and strike it does it ring? Yes, it still rings - it you used a high speed camera you could see the vibrations. But, the energy does not get converted into "sound" as there is no atmosphere. Instead this vibrating energy is converted into heat inside the bell. A similar situation happens in the hammer. Like you said, all heat - assuming we ignore the PE change of dust as the hammer strikes the surface...

With regards to the bell, the atmosphere carries energy away in the form of sound. We could say the atmosphere dampens the bell. Or stated another way, the bell would vibrate longer on the moon than on the earth.

Regards,

APD
• Why dont we need to cionsider the earth's orbit in these examples and in projectile motion? It doesnt matter right?
• I could be wrong about this, but I believe it's because we're calculating in relation to the earth.

If we introduced a third body then we'd probably have to factor in Earth's orbit and I imagine things would get much more complicated.