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!