why does a spacecraft need to go 25,000 MPH to break free of Earths gravity?
139268
One interpretation of the question is, why not just travel slower, continually applying thrust, e.g., a booster that goes at jetliner speed?  The reasons why that is impractical are:\n(1) you have to use a rocket since it must work beyond the atmosphere\n(2) with a rocket you carry all your propellant with you, unlike a jetliner which obtains reaction force from pushing back combustion products plus a large proportion of air\n(3) this means you don't have energy to spare \n(4) you have to put as much total (kinetic + potential) energy into the spacecraft whether you escape quickly or slowly\n(5) you want as much rocket impulse as possible to go into accelerating the spacecraft, and as little as possible spent just fighting gravity.  (Think what a waste it would be to hover on rocket power!)  This means high thrust for a short time to maximize kinetic rather than potential energy. \n\nSo you end up taking a modified "ballistic" approach, similar to firing the spacecraft from a cannon (which is all kinetic, no potential), but not so stressful and a little smarter.  You do all the accelerating during the first few minutes, then coast free of gravity.  You shape your ascent so your path is mostly vertical until you leave the atmosphere, to minimize energy wasted on air resistance.  In this phase you are providing some potential energy, but this can't be avoided.  Then you "level off" somewhat so your thrust isn't fighting gravity so much as you accelerate to escape velocity.
539111
There is a problem with the previous answer... the earth's escape velocity is around 25,000MPH... that number is based on an object without any propulsion, mankind has yet to develop a rocket or other propulsion system that can reach that speed... just wanted to clarify that ;)  we can't hit mach 30 (mach I is 720mph, 22,500mph is about mach 30) yet, but we can leave the earth's gravity field because of continuous propulsion (lots of rocket fuel)
837120
Mass accelerates more quickly in an intense gravitational field as compared to one that is weak.  The difference is in the time of acceleration.   On the surface of our planet, at sea level, a person accelerates at approx. 22 mph., but at the 0.717 mile location from the center of our planet a person would be accelerating at a speed 25 mph less than that of the speed of light.  Even though the mass does not physically move, the same physical properties exist as one moving in outer space at that speed.\n\nThe force of gravity is an energy factor, as found in the concept of c2 = E/m.  It is a product of a mass energy (heat) relationship.  This, of course, is obvious in that gravity performs work.
241909
The escape velocity is the minimum velocity an object needs to obtain in order to leave the surface of the Earth and never return, it will break free of the Earth's gravitational field forever.\nIt is not necesary to obtain escape velocity just to go into space.  While orbiting the Earth, you are most definatly still trapped within the Earth's gravitational field and you will eventually return to the surface.\n\nIf an object launched from the ground with an initial velocity lower than the escape velocity, the object will eventually (it may be take a very long time) return to the Earth and fall back down.
1037132
Well, you need to take more than one law into account.  There's also the law of inertia; that an object in motion tends to stay in motion along a straight line unless another force acts on it.  In the earth's case, that other force is the sun's gravity.  As a result of the earth's own intrinsic motion and the sun's gravitational pull, the earth continues to orbit around the sun (though the sun is winning out, veeeerrrry slowly).  To disturb that equilibrium, you'd need a very large mass and/or a very high speed to overcome earth's inertia enough to knock it out of orbit.  While the impact of asteroids do have *some* effect, because asteroids come from every direction their cumulative effect evens out.  \n\nAn analogy would be walking outside on a windy day.  Normally, if there's no wind or an occasional breeze, it doesn't affect you much.  But if there's a strong wind coming from one direction then you get pushed by it, and a really strong gust can knock you over.
1132892
I think the catch to your question is that "gravity would pull us away from the sun at a faster rate tus making gravity faster then light" is complete nonsense. Gravity is two objects pulling on each other. Gravity in and of itself doesn't pull anything anywhere, and it certainly doesn't pull two things away from each other, particularly if one of those two things has just exploded and no longer exists. If you are suggesting that the gravitational pull from the other planets would then pull us away from the ex-Sun, that might be, but the interaction between the remaining bodies in our solar system would be amazingly complex, not easily modelled, impacted by the force of the exploding star, and I see no reason at all to believe that it would cause the Earth to move out of orbit fast enough to escape the light coming from the ex-Sun.\n\nComplete gibberish.
414593
Hi Keenan!  Im Matt, an astronomy wizz.  If you look at most astronomy questions where I am, my answers are usually always the best of the best thanks to my power source, Blue Moons to Black holes By Melton Knocke.\n\nAnyway your question:\n\nEarth is moving fast, despite what people think.  It spins at about 1000 miles per hour, and goes 67 miles per hour around the sun.  You'd think that you would feel the spinning right?  WRONG!!!!  Sorry, cant help it.  Its so fun.  ANyway, the reason: All of the motions are almost completely constant. \n\nDon't get it?  Ever ride a train?  If you haven't, you can surely picture it as a car if you wish.  Notice how you feel abolutly no motion pretty much unless you speed up, slow down, or hit a bump?  Thats the same reason the earth never can be felt.  Its always going at a constant speed, never slowing unless it actually is hit with something dangerous enough to wipe out the human race.  \n\nThere is also another way in which we could indirectly feel the Earth's spin. Above we said that the Earth spinning is an example of almost constant motion. The reason we said "almost" is that the Earth's spin carries us around in a circle, not in a straight line. It's a very big circle, and it takes a long time to go around, but qualitatively it is the same thing that happens on a spinning amusement park ride, where it feels like you're being flung outward as the ride spins around. The spinning Earth is flinging us away from its surface a tiny bit, so that we weigh a little less than we would otherwise, simply because we are not being held down to the surface as tightly. \n\nAnyway, my answer is yours truly.  Enjoy!  Have fun.
862938
Simply? Gravity is the force responsible for accelerating you towards the earth's center. Thus, when considering the forces exerted during launch, there is a constant m(9.8m/s^2) force directed straight down to the center of the earth. Dividing all forces out by mass, then no matter what acceleration into the heavens the rocket is given, it will face a near-constant 9.8m/s^2 (~ closer to 8.6 at altitude (not that much different from on earth)). What this means pertains to energy.\n\nThe rocket must get form one level to another, so the energy well decreases, in the sense of its kinetic energy must at least be (GMm/r1 - GMm/r2), where r1 is on the ground and r2 is at altitude. If you didn't factor in gravity, no force would be acting downwards on the rocket. Technically, this means any acceleration could place it at any altitude; i.e., flicking it would put it in orbit.
170908
There are two forces involved to keep a satellite from falling down. First, GRAVITY is constantly trying to pull the satellite down, but at the same time the satellite's speed is trying to hurl it off into space--CENTRIPETAL force. As long as both forces are exactly balanced, the satellite won't fall down.
730409
The sun and earth exert equal gravitational forces on each other. If you ignore all the other interacting bodies, the sun and earth rotate about a common center of mass (which is actually located inside the sun). Contrary to what one answerer said, the earth is not in a "tenuous" or precariously balanced orbit. Our velocity in solar orbit is about 18.5 miles/sec. If we added (or subtracted) a bit of speed, say 1 mile/sec, our orbital shape would become more (or less) energetic and more elliptical. We might not like the climate change but we wouldn't fly off or crash into the sun. The velocity and the distance between the bodies determines the orbit. Change one or the other and you have a different orbit. Remove all velocity and you crash into the sun; add a huge velocity increment and you escape. (Note that if you're in a circular orbit you are traveling at escape velocity * 0.707, so that tells you how much faster you need to be going to escape.) This intrinsic stability enabled early experiments in putting objects into earth orbit before precise velocity control was available. The orbits weren't beautiful (and earth orbits will always decay due to atmospheric drag) but they were otherwise stable.
604528
Gravity won't let you. You have to go into orbit first, and 1 mph will not do that. You have reach escape velocity to escape earth's gravity, which is 11.2 km/s.
1170673
The Apollo missions that had to get to the moon had to hit 25,000 to get out of Earth orbit and for certain maneuvers around the moon. The space shuttle and/or ISS orbits the Earth at speeds between 17,000MPH and 18,000 MPH.\n\nUnmanned Spaceborn:\nHelios probes orbiting the sun hit 252,800 km/h in 1974.\n\nManned Airborn:\nOct. 3, 1967, the X-15A-2, piloted by Major William J. Pete' Knight, went Mach 6.7 (4,520 m.p.h.) above the Mojave Desert. Record stood until broken by the Space Shuttle in 1980. Highest altitude reached by X-15A was 354,200 ft.\n\nThe fastest air-breathing, manned vehicle, the SR-71, achieved slightly more than Mach 3 back in 1964. Chuck Yeager hit Mach 1 in 1947 with the X-1.\n\nUnmanned Airborn:\nRecently the unmanned speed record (in proximity to the earth anyway) was broken. The X43A hit Mach 9.6+. Scramjets are thought to be capable of hitting Mach 15+.\nexternal link\n\nManned Ground:\nMach 1.016 in the "ThrustSSC" by Andy Green in 1997.\nexternal link\n\nUnmanned Ground:\nA rocket sled hit Mach 8.5 (unmanned)
332344
you already know that as the Earth orbits the Sun it is weightless. But it certainly is not massless.\n\nBecause the Earth has mass it exerts a force of gravity and the magnitude of this force of gravity is determined by the mass of the Earth and the distance away from the center of the Earth another object is. Now, you know that if you drop something it falls. But did you know that as it falls it accelerates? That means that as it falls it falls faster and faster and faster until it hits the ground. On earth at sea level objects that fall speed up at a rate of 9.8 m/s every second. So, for every second something falls it falls 9.8 m/s faster with each passing second. You can see that at this rate you are going pretty fast real soon. (9.8 m/s is a little more than 20 miles per hour, so in three seconds you are already going 60 mph!)\n\nIf you go way high up a mountain the acceleration due to gravity is a little less than 9.8 m/s. If you go higher still the accleleration is even less and so on. You can tell how high you are by how fast objects that fall accelerate.\n\nNow, here is the really neat part: all objects accelerate to the Earth at exactly the same rate given the same distance from the Earths center. In other words, even the Moon is accelerating towards the Earth at a rate that is appropriate for its distance from the Earth's center! If another object, say with the mass of this desktop computer, were to be placed in the moon's orbit it would circle the Earth at exactly the same speed as the moon. Why? Because both objects are accelerating towards the Earth and all objects, regardless of their mass, accelerate towards the Earth at exactly the same rate.\n\nSo here is where it all comes together: If you know how far an object is from the center of a planet or a sun and if you know the rate of acceleration that that object is accelerating towards that planet or that sun you can easily calculate the mass of that planet or sun because only an object of that planet's or sun's mass can accelerate an object towards it at that rate at that distance.\n\nIsn't that amazing? This procedure was actually used to calculate the mass of the earth and it is used to calculate, well, the mass of any orbited body. In fact, we did not know the mass of the planet Mercury until we put a satellite around it.\n\nWhat is that magic equation? Well, there are actually several. Here is one that can be used of you know the time it takes an object to orbit the object you want to find the mass of (T) and if you also know the distance between the centers of both objects (r): M=4 pi2*r3/T2G. 'M' is the mass of the orbited object in kilograms; 'r' is the distance between the centers of the two ogjects in meters; 'T' is the time it takes the orbiting object to orbit the planet or sun you want the mass of and 'G' is the Universal Gravitational Constant (6.672 x 10-11 Nm2/kg2) and pi is 3.14. Note that all this tells you is the mass of the orbited object. This will not tell you the mass of the orbiting object. There is no way to calculate that without finding something to orbit it!\n\ncheck this website \n\nhttp://www.physlink.com/Education/AskExperts/ae170.cfm
475251
If I remember correctly from my high-school physics class, it's a combination of gravity and one of Newton's laws of motion - the one that says "an object in motion stays in motion unless acted on by an outside force." So that force keeps it moving around and gravity keeps it from just flying off into space.
895548
Minimum speed for what?\nIf you want to leave earth orbit then you need to be over the earth escape velocity.
1033188
Yes they do.\n\nIsaac Newton provides the answer. He phrased three very important laws.\n1st: A body will remain at rest or at motion with a uniform speed unless it is acted on my an external force.\n2nd: The acceleration of a body with a force acting on it is that force divided by the mass of the body (F=ma)\n3rd: Every action has an equal and opposite reaction. \n\nIn space, spacecrafts are not standing at a halt but they are moving constantly with a constant speed. The can keep that constant speed because of lack of friction. \n\nEven the satellites that remain on top of a certain spot on the earth's surface (geostatic satellites) are moving along with the rotation of the earth.\n\nThe third law answers your question. To change the movement of an object you must put a force on it. So if a spacecraft needs to change position it has to burn fuel to produce kinetic energy and move.\n\nOur experience on earth says that everything eventually stops moving but this is because of friction but there is no friction in space.
1041594
Things that are orbiting our planet are in a state of constant "free fall". That is why they appear to be weightless. The force of gravity is still very high on the astronaut.