(continued from Part 2)

“Orbit,” Gregory interrupted. “I get it.” Gregory sat there silent, his front teeth biting his lower lip. Suddenly he jumped to his feet. “So, if the football is like a space craft, with people inside, the space craft and people will be falling toward the earth, but because they are moving so fast toward the horizon, the Earth curves downward just as fast; they will never hit the Earth. Since the spacecraft is not pushing on them, they will seem weightless.”

Mr. Jackson held his hand up, palm forward, just in front of his son, and Gregory gave him a high five. Then his eyebrows lowered, and moved closer together. “If only my boss would trust me like you do.”

Gregory wasn’t listening. “So the rocket doesn’t just lift them to two-hundred miles altitude…”

“That’s right son,” Mr. Jackson said. “The purpose of the rocket is to create both speed and altitude, kinetic and potential energy. you remember all those Cape Canaveral launches? Right after they cleared the tower, the rocket began to pitch over and fly east, out over the ocean. As they accelerate upward, they also accelerate down range…”

“Toward the horizon!” Gregory concluded.

“By the way, do they launch eastward to avoid flying over the cities?”

“That’s part of the reason. They fly east from the east coast to take advantage of the Earth’s rotation direction. This free speed saves a lot of launch fuel or cost.”

“Really?” Gregory scratched the back of his neck. “How fast is the ground moving at the Cape?”

“I’ll tell you what, later tonight, you figure it out using Earth spin rate, Earth diameter, and the local latitude. Farther south at the equator, the ground speed is approximately V equals R times omega (V=Rω), where V is the ground speed, R is the Earth radius, and omega is the Earth spin rate in radians per second. You’ll have to convert R and ω to the proper units to get V in miles per hour (mph).”

As they returned to the house Gregory looked up at his father, “How fast does the space craft need to go to stay in orbit?”

“Faster than a speeding bullet,” Mr. Jackson quipped. “That depends on the actual altitude. Orbiting at about two, to two-hundred and fifty miles above the surface of the Earth requires a speed of about seventeen thousand five-hundred miles per hour.”

Gregory stopped and looked up at his dad again. “Wow.” After a moment of silence Gregory continued, “So, the main rocket isn’t just to blast the space craft up to two hundred miles altitude…” He paused a moment. Mr. Jackson listened.

“The rocket is to get the craft going fast enough to stay in orbit, fast enough to miss the Earth as it continues to fall toward it.”

Mr. Jackson gave his son a football-player slap on the back.

“Oh no.” Gregory blinked rapidly.

“What’s the matter?”

“Jenn was right. What am I gonna say to her?”

Mr. Jackson laughed.

Gregory went upstairs to return to his biology homework. Half way up he stopped, turned around and called back to his dad who was still holding the football. “So dad, what are you making at work right now, and what’s that contraption you’re building in the garage?”

“There’s a lot of red-tape at work.” Mr. Jackson lifted his chin and smiled. “I’m getting a head start here where it’s easier. I’ve figured out a way to…”

“A head start at what?”

“I’m inventing a method to measure the weight of weightless objects in space.” He grinned, tossed the football a foot in the air and caught it.

“But,” Gregory looked down at his feet, then lifted his head again, “if things are falling, how can you weigh them in space?

(To be continued)