Misconceptions about Satellites and Weightlessness.
A common thread in this set of examples is a lack of understanding that an Earth's satellite is basically a piece of matter that moves under a force of Earth's gravity, so it falls. Specifically, if you stand in your backyard and drop a rock, it will fall straight down. If instead you toss the rock sideways, it will fall down at the same rate as before, while moving sideways at a constant speed, so the path will be curved. (This assumes that somehow the atmospheric drag is not present.) Now, it you toss the rock horizontally at a big enough speed, the curvature of the rock's path will match the curvature of the Earth. So, the rock keeps falling and yet won't hit the ground, because the ground curves away. You'd have a satellite!
So why do we need rockets? For two reasons: First, there is the atmospheric drag, so one would have to toss the satellite from above the Earth's atmosphere. One could be to do it from a top of a very tall tower, but the tower would have to be at least 100 miles tall to stick above the atmosphere. Second, the satellite needs to move fast, about 8 km/s (5 miles per second), and you can't pitch that fast.
éBack
to the Table of Contents
"Sometimes, we talk about weightlessness, when a person is in space, for example. They aren't really weightless, however. There is another force, the force created by the rocket's traveling around in orbit. This force balances the force of gravity."
Physicist's comments: There is no other force. The only acting force is the gravity. See the explanation above.
éBack
to the Table of Contents
"An orbiting solid object, like a bit of chondrite, maintains a simple balance between centrifugal force (directed outward) and the Sun's gravity (inward)"
Physicist's comments: I was somewhat surprised to find this error, as the Sky and Telescope magazine is a reliable source of information with high editorial standards. But apparently this one slipped through.
Again, the only force acting here is the Sun's gravity, so the object in question is in a free fall towards the Sun. See the explanation above.
éBack
to the Table of Contents
Question to Marilyn: "Earth's gravity holds spaceships in orbit, but the things inside them are weightless and float around. Why doesn't gravity have an effect inside?"
Marilyn's answer: "When a space shuttle is orbiting the Earth, the sum of "downward" (gravitational) force and the "forward" (inertial force) of the moving ship and its contents nearly equals zero. So both the ship and its contents are in free fall, which makes everything weightless. They stay in orbit while they're "falling" (being pulled toward the Earth) because the inertial force (centrifugal force, in this case) of the moving vehicle is radial - away from the Earth."
Physicist's comments: Apparently, Marilyn does not understand what a free fall means, so her "explanation" does not make any sense. First of all, as we all know from a high school trig, it's impossible to add two perpendicular forces and get a result that's "nearly" zero. Fortunately, one does not have to add any forces, because the only force acting on a shuttle and its content is the "downward" gravitational force of the gravity. There is no "forward" force, and likewise there is no "centrifugal force". If you stand in your backyard and drop a rock, it will fall straight down. Now, let's assume that there is no atmosphere on the Earth. If you tossed the rock sideways, it will fall down at the same rate as before, while moving sideways at a constant speed, so the path will be curved. Now, it you toss it horizontally at big enough a speed, the curvature of the path will match the curvature of the Earth, so while the rock keeps falling, it can't hit the ground, because the ground curves away. You have a space shuttle in a free fall. If you replace the rock with a coffee can and some pebbles inside, everything will be falling at the same rate, so the pebbles inside won't move with respect to the coffee can. Now, if you stand on a bathroom scale in an elevator and the cable suddenly snaps, everything will start falling toward the ground, and the reading on the bathroom scale would drop to zero. As you fall towards the ground, the gravity keeps pulling on you, but the reading on the scale is zero, because the scale falls too. This is what is meant by being weightless. I do not like this term, because it's in fact misleading - your weight (gravitational pull on you) has not changed, it's the reading on the falling scale that is zero. I'd prefer to say that your "apparent weight" is zero.
éBack
to the Table of Contents
"(...) the shuttle blasts off (...) Then comes the tremendous pressure of three G's and the sudden release into weightlessness as the ship leaves the gravitational field behind."
Physicist's comments: Ouch! See the explanation above.
By the way, how big is the pull of Earth's gravity on an astronaut, whose weight on a launch pad is 220 lb? The force of gravity decreases as a square of the distance between the center of Earth and the astronaut, and the Earth's radius is 6380 km. Let's assume that the shuttle is in a circular orbit 200 km above the ground, i.e. 6580 km away from the center of the Earth. Then (6380/6580)2 = 0.94, so the Earth's pull on astronaut in orbit is 0.94 x 220 lb = 207 lb, and his/her acceleration towards the center of the Earth is 0.94G = 9.2 m/s2.
éBack
to the Table of Contents
"... the orbit of satellites ... decreases over time because of atmospheric drag on the spacecraft. They drop a bit more than half a mile every year, and twice that during some years. As the satellite slows down and drops..."
Physicist's comments: The error here is a subtle one. The correct statement should read: "As the satellite speeds up and drops.."
That's because the air drag causes the satellite to drop, and satellites in lower orbits have greater speeds (keep in mind that a shuttle's orbital speed is some 7.8 km/s, while Moon's orbital speed is only 1.0 km/s). Paradoxically, the presence of atmospheric drag results in an increase of satellite's kinetic energy. If you wonder about the energy conservation - don't! As the satellite drops, it's gravitational potential energy decreases twice as fast as kinetic energy increases, so overall, the total mechanical energy of the satellite decreases.
éBack
to the Table of Contents