let it be known that because the oceans slosh around on Earth as it
rotates about it’s axis (these are called ocean tides), the speed of
Earth’s rotation is gradually slowing down. Billions of years ago a day
on Earth was only about 8 hours, and now it continues to slow down. We
compensate this twice a year by adding leap-seconds as needed.
slowing of the earth’s rotation causes the moon to drift further away
from us every year by about 1.5 inches, so eventually the Moon will
escape Earth altogether and drift away.
It just moves forward. The moon isn’t stationary. It has inertia and no atmosphere in space to slow it down. It just goes. As it moves forward, the gravity of Earth tugs on it and it falls, but because it’s moving forward it misses Earth as it falls and goes around, again and again. That’s called an orbit.
Yes it would return to Earth. The escape velocity for Earth is more than 11 km per second or 33 times the speed of sound. This is about 9 or 10 times faster than a rifle bullet.
So even without air friction the bullet’s going to go up, gradually slow down, and fall.
The problem is that without friction it’s going to come down too fast and kill someone at roughly the same speed as when it left the gun barrel.
In reality if you shoot a bullet into the air, when it drops it meets air resistance and winds up stabilizing at about 30–40 miles per hour. That’s enough to hurt you, but not as fast as if there was no friction.
Let’s revisit Sir Isaac Newton’s cannon.
In the above mentioned article we had placed a level on the cannon and shot the cannonball at 1440 feet per second parallel to the ground until it fell and bounced and rolled and came to a stop.
We didn’t want to shoot it up in the air in a big arc to see how far we could get it to go because we knew if we shot the cannonball up in the air in an arc, it would go much further. Naturally. We were simply interested in the effect of gravity on a projectile as it moves forward and we chose to level the cannon and shoot straight out.
So now let’s do just that, but because we know the cannonball can only go so fast, we’ll do it with a rocket. We’ll shoot the thing up in the air into a big arc and see where it comes down.
Roar! We launched. The burning rocket fuel accelerates our rocket faster and faster until it’s going 20 times the speed of sound. That’s Mach 20. Pretty lickedy-split I dare say. It’s going faster than any jet plane that I ever flew in went. In fact, it’s going so fast that every second it travels almost 5 miles (4.9 miles to be precise). I’d be scared out of my wits if I was riding that thing.
Okay, so this rocket just goes so fast and so far, and then you know what happens? Well, it runs out of fuel of course. It can’t burn forever. We only put so much of that rocket fuel inside of it, so once we light it and run to the side and it roars to the sky it’s going to run out of fuel after awhile.
So now here it is, way up in the sky. It went so high, so fast, that it actually went higher than the air. The sky around the rocket went dark, the stars came out and it got really quiet.
Yet it’s still going forward in that big arc even though we’ve run out of fuel, because essentially we threw something up in the air really fast.
Technically we didn’t even need rocket fuel, we could have used a slingshot if we could have achieved that speed. Alas, trial and error has shown me that unless you give it that extra boost as it goes up, it’s never going to go 4.9 miles per second. I’ve been through a lot of slingshots as a kid and never managed to shoot a projectile going that fast. Good thing, too! My neighbors would have been upset.
As our burned out rocket shell continues to move upward, it’s going to slow because of gravity. After all their’s no more rocket flames to boost it any higher. When it reaches the top, we call that the Aphelion, it levels out and slowly starts to curve down toward the Earth. Wherever it hits, somebody’s going to get really upset.
Except it’s so high and going so fast, that as it arcs downward toward the ground, accelerating faster and faster, it completely misses the planet! It just goes zooming right by planet Earth, it’s course warped by gravity, and whipped all the way around to the other side and then flung out into space again.
Marvelously, this happens again and again, much to our amazement. It doesn’t look like that rocket’s going to crash down at all! It’s in what they call an elliptical orbit.
That my friend, is another example of what keeps things in orbit. The speed one needs to achieve orbital velocity is 4.9 miles per second, or 7.9 kilometers per second. At that speed things go up, then free-fall to the earth, miss the earth, and keep going round and round.
Sometimes the orbit is highly elliptical, or if you’re very clever, you can make it almost round. The Earth’s orbit around the sun is elliptical, and the moon’s orbit around the Earth is also elliptical (that’s what a super moon is all about, when the moon is both full and closer to the Earth).
I’m sorry, Sir Isaac. Your cannon didn’t cut it this time. Had to make a rocket. If you want to see how this works, click here!
The Earth is falling into the sun and the moon is falling from the sky.
Once upon a time, Sir Isaac Newton had a cannon. Noisy thing that. Orbitologists call the figurative device “Newton’s cannon.”
I managed to borrow one of these cannons from the National Museum of Orbitology and Conjecturism, and am standing next to a large pile of cannonballs (see image).
Depending on all sorts of factors, like how much gunpowder, how big the cannonball, etcetera, I’ve discovered the cannonball leaves the cannon around 1440 feet per second. Alas, after leaving the cannon, the speeding cannonball gets affected by wind, gravity and distance it has to travel.
The first factor is the wind resistance. At 1440 feet per second on a calm day, the spherical ball of lead immediately encounters 1440 feet per second wind resistance in the opposite direction of flight. The cannonball’s gonna slow down.
The second factor is gravity. No matter how fast that thing travels through the air, it’s still going to fall toward the ground for the same reason we plant our feet here. Gravity.
A third factor, I suppose, is if we’re shooting at a target or just an open space to see how far the ball will travel. Since I want to see how far the ball will go, I’m in an open field.
A fourth factor is trajectory. Am I shooting level to the ground, or in a big arc? Obviously it’s going to travel farther if I angle the barrel of the cannon up and shoot the ball in a big arc. So what to do? Let’s just put a level on the barrel and shoot it parallel to the ground to see how far the projectile is going to travel before wind resistance and gravity pull that hurdling ball of metal down to the unyielding ground below and then it bounces and rolls until it comes to a halt.
Now that we’ve fired the cannonball, let’s take a tape measure and see how far it went before it hit the ground. Surprising distance it seems for such a big heavy object. I wonder if we could make it go any farther?
This is the example of Newton’s Cannon. Given a condition of no atmosphere, and enough speed, that ball would travel around the curve of the earth, hitting further and further away from us. Speed it up even more and it will never hit the ground (see illustration).
That’s an orbit.
Umm….wait. Something’s seriously wrong here. My brain puts up red flags. I’m a layman, not a real scientist, so what do I really know about physics and all that stuff. I’m a buff isn’t it enough? Seems to me that a falling object should accelerate, not just fall at a constant speed. Seems to me the rate of acceleration of a falling object is (Googling it now) 9.8 m/s/s. “Free-falling objects are in a state of acceleration. Specifically, they are accelerating at a rate of 9.8 m/s/s. This is to say that the velocity of a free-falling object is changing by 9.8 m/s every second.” –physicsclassroom.com
So the cannonball should not just curve around with the curve of the earth, but accelerate downward as it falls, thus never achieving orbit and always hitting the ground. Therefore nothing can orbit anything and the moon can’t stay in the sky and the Earth is going to fall into the sun. The International Space Station is doomed tomorrow and all the GPS satellites are going to fall down. Forget about Dish Network, DirectTV, SeriusXM Radio, weather satellites and Google Earth. It’s all coming down.
Well, obviously wrong, but why wrong? This is a question this layman has pondered over many an hour sitting in pondering places at various pondering moments in this pondering life. I suppose I’m going to have to just ponder up another Google search. I will ask this question another way posted here, and after you read that one you can read here and then here!