VERY URGENT: Why doesnt.....

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tbone12

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Hello, I am desperate for the answer to this question:<br /><br />Why dont planets and satellites, such as the earth and the moon, get sucked in towards the sun from its gravitational pull? Any help or leads would be greatly appreciated.
 
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CalliArcale

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No problem. <img src="/images/icons/wink.gif" /> The reason is because they are moving forwards very fast.<br /><br />Basically, the Sun is constantly pulling the Earth towards it. If the Earth were motionless relative to the Sun, this would cause the Earth to fall directly into the Sun! But fortunately this is not the case. Earth is travelling forwards at a fantastic velocity of about 30 km/second! Now, we know from Newton that a solid body will tend to travel in a straight line, right, except when acted upon by an outside force? The Sun's gravity exerts an outside force on the Earth, deflecting the Earth from a straight trajectory. As the Earth moves forwards at 30 km/sec, the Sun pulls the Earth towards itself. But because the Earth is moving forward so fast, it ends up missing the Sun. It is essentially falling around the Sun. <img src="/images/icons/wink.gif" /><br /><br />That's what an orbit is -- falling around an object. You're only in orbit if you're going forwards fast enough that you miss the object you're orbiting. Otherwise you're on a suborbital trajectory, which means you're not going to miss the object: you're going to hit it, probably quite painfully. <img src="/images/icons/tongue.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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yevaud

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This is why we're here. Calli probably more than most. <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>
 
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dark_energy

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Now you can warn your friends about the moon accelerating towards the earth. My grade 11 physics teacher frightened me like that. <div class="Discussion_UserSignature"> </div>
 
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yevaud

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Actually, the moon is slowly moving farther away from Earth.<br /><br />This is very topical (intended for a young audience), yet it conveys exactly what I mean.<br /><br />Moon's Orbit <div class="Discussion_UserSignature"> <p><em>Differential Diagnosis:  </em>"<strong><em>I am both amused and annoyed that you think I should be less stubborn than you are</em></strong>."<br /> </p> </div>
 
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dark_energy

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The moon accelerates towards the earth at a rate of 0.00272 m/s^2 because of the earth's gravitational pull on it. Granted I do not know whether it's moving farther away, but I do know that it is <i>accelerating</i> towards it. The moon would not have a deceleration from the earth. <div class="Discussion_UserSignature"> </div>
 
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CalliArcale

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You are correct -- there is an acceleration towards the Earth. Yet the Moon gets no closer to the Earth and is in fact receeding. This seems totally absurd at first, but this is actually because most people use the word "acceleration" in a way only suitable for everyday life on the surface of the Earth, where we labor under the myth of a universal frame of reference. <img src="/images/icons/wink.gif" /> In fact, acceleration is any change at all to your velocity vector -- which includes the direction in which you are travelling. So if a force deflects your trajectory, you are accelerating. If the force is removed, the acceleration is removed, and you are now travelling in a straight line, just not the same one you were originally on.<br /><br />It's all ballistics, basically. Fire a rifle. If we pretend the air has no actual effect, the bullet doesn't slow down at all, relative to the ground. But just like any loose object without a source of propulsion, once its out there in the air, it is accelerating towards the ground. Interestingly, it will fall to the ground (assuming the ground is flat and there are no obstacles for the bullet to hit) in exactly the same amount of time as if you'd dropped it out of your hand at the same height as the gun's muzzle. It is dropping just as fast and accelerating just as much towards the ground regardless of whether it was motionless relative to the Earth or fired parallel to the ground.<br /><br />Of course, the reality is that the Earth isn't flat. If your gun is sufficiently powerful, you will find that the faster the bullet flies, the more time it takes to hit the ground. This isn't because it's flying or anything. This is because by the time it falls that much, the ground is further away -- the Earth is sloping away underneath the bullet. Extend that far enough in your mind and it's easy to see how orbits work -- get the bullet going fast enough, and it will never hit the ground. <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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