How best to explain to a group of average school kids that ZERO GRAVITY does not exist and what space travellers experience is FREE FALL.
Free fall seems a little hard for them to understand .
thanx
Indian
You could show them clips of people inside the “Vomit Comet” airplane, experiencing ~20 seconds of free fall. That might get the idea across that gravity isn’t magically disappearing once you leave the Earth’s atmosphere. If the age group is appropriate to understand the concept, you could also explain about proposals for an orbital tower or space tether, and explain that you’d still feel weight on one until you got all the way to geosynchronous orbit.
You’re going to have a hard time doing this.
Not because there is anything wrong with your question phrasing or assumptions, but because most of the definitions of zero gravity are actually about the appearance of zero gravity. Doing a Google search, I got a ton of definitions like this:
What they are talking about, though, to my poor understanding is what you (and I) call free fall, or microgravity.
I have read that gravity itself is pretty much like any other force–electromagnetic, weak nuclear, strong nuclear, etc., in that it tapers off and off and off but, like our atmosphere, doesn’t really ever stop. It just gets to the point where we can’t measure it any more.
Others will doubtless come in to correct me if I am wrong in this, no worries.
I think this a good question, but be careful about sending them against “what everyone knows” without a lot of ammunition. We really need a real physics-type guru to do a better explanation than my poor attempt here…
Piece of cake. Instead of starting with the concept of “ZERO GRAVITY” which is pretty mind-boggling if you try to grasp it*, start with the concept of “FREE FALL”.
Have them imagine skydiving. Or show videos. Whatever.
The only visual difference between a skydiver and an astronaut is the wind whipping at the skydiver, so get them to disregard that. Suddenly, they see that gravity has little/no effect on a skydiver. They grasp the concept; you become their hero; mayor gives you key to city; etc…
*(What do you mean ‘zero gravity doesn’t exist’? What’s your definition of “zero gravity”? By your definition, is a person floating at the center of the universe, who has equal mass on all sides, NOT at zero gravity? Or are you just trying to say that “zero gravity” doesn’t exist in Earth’s orbit. Heck, by some definitions, “zero gravity” exists whenever you jump.)
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GQ > IMHO
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Urk? Any skydiver who ignores the concept of “gravity” does so at his own peril. At any rate, skydivers (at least at normal skydiving altitudes) are not in freefall; when you jump out of the plane you are already at terminal velocity (albeit in the forward direction due to the motion of the plane) and are rapidly at terminal velocity in the downward direction. (You can change velocity somewhat by flattening out are arching further, but not massively.)
From the standpoint of a physicist (or, at least one studying General Relativity) there is no difference between zero gravity and freefall. Both are experiencing no net force or thrust, and thus are in an equivilent reference frame (with a few small quibbles that are not of issue for calculations short of seven or eight decimal places). From an outside observer, the difference is that the bloke in freefall orbit about a planet is under acceleration (although moving at constant speed if in a circular orbit) and is constantly changing direction, while the dude in zero gravity is moving in the same direction at constant speed. This is a subtle point and probably requires a more inclusive understanding of physics than average school kids typically display.
For your purposes, you can show that the astronaut in freefall orbit around a planet stays in orbit and the only possible influence keeping him there is a non-negligable attraction of gravity, so being “in space” is not the same as zero gravity (and in fact in the orbit of the ISS the actual attraction of gravity is a large fraction of sea-level gravity). The implication of this that is frequently not illustrated is that a felt “force” is actually the interaction between two opposing forces, like gravity and the electrostatic repulsion of the ground against your feet, or thrust from a jet engine versus air resistance. This is a basic conceptual point that anyone studying science should understand but seems to escape even many physics undergraduates.
Stranger
Heh, point taken.
I maintain, however, that even though sky-diving isn’t actually free fall, it is still a useful tool in teaching the concept to school kids. If you want to make it more apt, remove the atmosphere.
Ummm…just politely asking, but what is it, then, that is causing the skydiver to accelerate towards the earth below?
Pick up any small object and tell them to imagine a tiny person riding on the back of the object. Tell them watch closely and let it drop to the desk or floor.
Now explain the following:
If there had actually been a tiny person riding on that object as it fell, they would have felt the same “lack” of gravity as the astronauts feel in space. Back that up by showing pictures of the “Vomit Comet”.
But obviously the object was in the same normal gravity that the students were in the whole time and never in “zero gravity”, as evidenced by the fact that no one was floating out of their chair during the demo.
So free fall and the feeling of no gravity can easily occur in a perfectly normal gravity field.
In order to actually be in zero gravity, you’d have to be so far from any object that their was no measurable gravity field at all. Back this up by showing pictures of galaxies and clusters of galaxies that are way the hell and gone from each other, and yet the stars are still held in groups and clusters by their gravity. You just can’t (practically, anyway) get far enough away from everything to be a truly zero gravity place.
They may be old enough to have been on roller coasters and a common technique for most coasters is to provide “negative g’s”… you crown the top of a hill in the coaster at a fast speed and fly out of your seats (unless the restraints are in place).
Now tell them that the speed and the hill are carefully matched so that you don’t fly up or fall down - you are “weightless”.
Next extend the analogy to a hill that never ends and that would be an earth orbit.
Thanx a lot for suggestions… Looking for more inputs.
I get a lot of queries from curious kids and I want to be sure on what I tell them…
When I taught physics, I covered the concept of orbital motion and free fall using Newton’s cannonball.
Once they get the idea that a cannonball in freefall experiences weightlessness, you extend that to the idea that a cannonball in orbit is constantly falling, which is why objects in orbit experience a feeling of weightlessness.
For freefall closer to Earth, I think that a falling elevator is a better example than skydiving. Skydiving is problematic because every picture or video students have seen of skydivers clearly show how omnipresent air resistance is. Air resistance is less of a perceived issue with a falling elevator. Also, many students have ridden amusement park rides where they experience “freefall” or something close to this, such as those towers with seats that drop.
I agree that you need to emphasize that gravity does not magically turn off for objects in orbit, and that gravity is actually keeping the object in orbit. As Stranger mentioned, for objects in low Earth orbit (LEO), approximately 500 km up, the force of gravity is about 85% of that on the surface of the Earth, certainly not near zero.
The concept of microgravity, at this stage of discussion, generally just confuses students, IMHO. Bringing up microgravity inevitably leaves some students with the erroneous impression that the force of gravity in LEO is some tiny fraction of that on the surface. Microgravity is actually a manifestation of orbital tidal forces. I wouldn’t bring up microgravity unless a student asks about it.
Secret rocket boosters.
But really: it’s all about the frame of reference. To the skydiver himself, and especially to the guy with the camera who is filming him, there is no evidence of gravity’s effect until he pulls the cord.
If you remove the atmosphere (to prevent terminal velocity from arresting downward accelleration), then the skydiver (presumably in a space suit, since there’s no atmosphere) will look, act, and feel exactly the same as an astronaut on a space walk. Until the skydiver pulls the chute or hits the ground.
ETA: Robby’s elevator is a fantastic idea too, that accomplishes the same thing.
I think kids may not understand outright how something in orbit could be constantly falling if its not going “down”, so if you gave them a basic primer on why things orbit, and what’s going on to cause it, and then follow with robby’s suggestion, you’ll see a lot of light bulbs in the classroom
I second this. In fact, in the Prudential Center in Boston one night, we took an elevator from near the top to the ground floor (no stops) and I swear it was falling fast enough to give me the feeling of reduced weight. It was very cool.
It’s not the fall that kills you. It’s the sudden stop at the end.
How’s this for a basic explanation?
At this point I’d draw a picture of the earth (which with my artistic skills is just a circle labeled earth). Draw one arc showing the path of a projectile leaving earth and crashing back into it. Then draw another arc that goes a little farther before returning to earth. Then draw one that goes so far that it misses the earth and instead circles all the way around it. In this way you can show that being flung into the air and falling back down is no different than being flung into orbit – orbiting is just like falling but with a greater initial fling.
One potential point of confusion: Even when you’re moving away from the earth, or keeping a constant distance from it, you can still experience “zero-G”. It’s not your velocity that matters, it’s your acceleration. Depending on the age of the kids the distinction between velocity and acceleration might be a bit subtle. If anyone asks, I might just remind them that in are elevator example the important thing is that your speed and the baseball’s speed change in the same way. In that example they move down and get faster, but if they’re moving up and getting slower it’s not much different.
Also depending on the age of the kids I might not start with “The force of gravity exerted on you by the earth.” I’d start with “When you jump, you don’t just fly away. The earth pulls you back down with an invisible force called ‘gravity’. How hard it pulls depends on how far away you are from the earth …”
It might also help to include some cartoon style pictures for the elevator part. Sort of a before-and-after. The first “before” is a guy in an elevator (with a cord coming out the top), holding a baseball. In the “after”, the elevator is at the same height, but the ball is on the floor, with an arrow from its old position to its new one. In the second “before”, the guy is in an elevator holding the ball with a severed cord coming out the top. In the second “after”, the elevator man and ball are all further down the blackboard (or whatever), with arrows from their old positions to their new ones. But the elevator is still at waist height of the man (or whatever height it started at).
Visual aids are useful because some kids learn more visually and others learn more from listening.
I’d use the Newton’s Cannonball pictures. That uses Same-earth, different-cannons. Back that up with a same-cannon, different-earth drawing. First, you use a baseball and a flat line representing the ground. Say it’s from the mound to home plate. Draw the path of the ball. Then tell the kids you’re “zooming out” and make the ground curve just a little. Say it’s from the shoreline to the horizon. draw the path and show it meeting the ground a little further out, since the ground curved away a little. Then “zoom out” again, and use a semicircle. At this point, curve the path of the ball the same way but make it never touch the earth. Between Newton and the zooming one, they should get it.
Make sure you make two points clearly. In the close picture, gravity pulls downward. In the earth picture, gravity is just pulling toward the middle, not down. Try throwing in the phrase “The astronauts are falling toward the ground, but the ground is falling away from them.” They won’t (shouldn’t) take you literally, and that might get the few slow ones to understand.
Well, normal airspeed on jump run would be something like 60 to 70 mph; terminal velocity for a skydiver falling belly-to-earth is around 120 mph. After exit it takes around 10 seconds to reach terminal velocity.
I don’t agree. At terminal velocity, you have no sensation of weightlessness. Nor should you, since your velocity is constant.
Agreed - in this case, he will certainly have the sensation of weightlessness. But deploying a parachute isn’t going to do much good.