Always Come Down
| What Goes Up Doesn't Always Come Down | |
Or everything you always wanted to know about microgravity. . .
Fact One:
Everything falls at the same rate no matter how much it weighs.
If you lift something up above the ground and let it go, it falls straight down toward the center of the Earth at the same rate of fall no matter how much or how little it weighs (air resistance helped to hide this fact about nature for a long time). Use the Paper Drop activity to demonstrate this fact.
Fact Two:
Objects in motion that have no forces (force is defined as a push or a pull) acting on them move at a constant speed in a straight line forever. If an object is not moving, it remains motionless forever unless some force acts on it.
This is known as Newton's First Law of Motion or the Law of Inertia.
About 400 years ago a man named Galileo wondered about the "natural state" of earthly objects. What do objects do when no forces act on them? For thousands of years the answer to that question seemed so obvious that it was hardly worth asking. Everything naturally comes to rest and stays that way until something moves it! This seemed like a stupid question, but was it? If you give something a push and then stand back and observe its motion, it will eventually stop moving (in physics we say that it will come to "rest"). This caused people to believe that the natural state of an object was to be at rest (of course, the sun and the moon were heavenly objects and so they were not subject to this earthly rule of motion). But Galileo wasn't satisfied with this. He asked himself some really good questions and then went about observing nature for the answers. He noticed that the more he waxed a wooden surface the longer an object would move before coming to rest. He wondered what would happen if he was able to get the wood perfectly smooth. Would the object ever stop moving? What happens when you push a hockey puck on a rough surface like a road? On ice? Do you see what Galileo was thinking? Perhaps the reason that things always stopped is that a force was acting on them, and he realized it was the force that we call friction. Friction is very hard to get rid of, and so it is no wonder that it took humans so long to realize their mistake. About 100 years later, Sir Isaac Newton generalized that when no net force acts on an object, it continues to do whatever it is doing (either remains at rest or moves at a constant speed in a straight line). We now refer to this as Newton's First Law of Motion (also called the Law of Inertia). Use the Air Puck activity to demonstrate this law.
Fact Three:
Horizontal motion is independent of vertical motion.
Now what does this mean? Imagine this situation: you drop a ball (A) above the ground, and at the very same time and from the same height, you throw a ball (B) out to the side parallel to the ground. Now as you think about this situation, ask yourself which one of the balls will hit the ground first? Look at the diagram below to help you understand the question.
Would you believe that they will both hit the ground at the same time, and it doesn't matter how fast you throw ball B? Use the Projectile Motion activity to demonstrate this fact.
The demonstrations help us to understand gravity and what is happening to our astronauts. A review follows.
First, we know that everything falls to the Earth at the same rate.
We also know that objects stay doing whatever it was that they were doing if no forces act on them.
If you throw something parallel to the Earth's surface, it falls at the same rate as an object that is dropped from the same height, and they both hit the ground at the same time.
If you lift an object off the ground and then let it go, it starts to gain speed so you know that a force must be acting on it. (If no force acted on it, it wouldn't be able to move since it was "at rest" in your hand before you dropped it, and things can't change what they are doing unless a force acts on them.) Of course, you already know what the name of the force is that acts on things that you drop above the surface of the Earth! It is called gravity. The force that we call gravity is a pull that is always directed straight toward the center of the Earth. If the Earth were flat like some people used to believe, no matter how fast you threw something out horizontally, it would hit the ground. The faster you threw it, the farther away along the ground it would hit.
Throwing the ball harder (in a direction parallel to the ground) only increases the distance along the ground that the ball travels until it hits. It does not increase the amount of time that the ball has until it hits. All of the balls will hit the ground at the same time. But (and this is a very important BUT) the Earth is not flat! As something falls "straight" toward the center of the Earth, it has to curve around with the Earth (when an airplane flies around the Earth, it stays at the same altitude above the surface. It, too, curves around the Earth, yet you feel like you are in level flight).
Well, we now have all the pieces of the puzzle to make sense of this strange condition called orbit. The final diagram is to help you understand orbit. A very similar diagram was first penned by Sir Isaac Newton more than 300 years ago.
The fact that the Earth is a sphere affects how far the object travels along the ground as the object falls. If you match the horizontal speed of the object with the curve of the Earth so that as it falls it winds up back where it started from, the object is in orbit! And that is what is happening to the astronauts. They are always "falling" toward the center of the Earth, but as they are falling, the Earth's surface is curving. They never hit the ground unless they slow down, which is exactly what they do when they want to land.
This means that the astronauts are not "weightless" at all. In fact, it is their weight (or the Earth's gravity) that is allowing them to orbit in the first place. If they were weightless, they would leave the Earth for good and never return. It would not be possible to orbit without gravity. Yet we have all seen the pictures of astronauts floating around in space. So why do they look like they are weightless? As in the illustration, they are indeed falling toward the Earth but so is everything around them including the space shuttle and their suits, etc. Imagine yourself in an elevator when the cable breaks. You plunge toward the Earth but so does the floor and the walls and the pen in your hand. Before you hit the ground with disastrous consequences, you would feel just like the astronauts; you would seem to "float" in the elevator until you hit the bottom. You could let go of the pen, and it, too, would seem to float there in front of you just like the astronauts. You may have either ridden or seen the amusement park ride, called free fall, that takes you to the top of a tall tower and drops you. This is the same idea. NASA uses such free fall towers to mimic orbit for testing equipment destined to go into space. The condition in which all your surroundings are falling at the same rate as you is called free fall. To help explain free fall, demonstrate Free Fall in a Cup.
So, why then, did this strange condition, in which the astronauts are in free fall around the Earth, become known as microgravity?
It indicates to you that gravity is not eliminated, but the overall effect of gravity is not apparent to you as you fall. The astronauts are not living in a gravity-free world, but they seem to "float" because everything surrounding them is falling at the same rate.
The word "micro" is Greek for very small (literally, it means one-millionth), and so the term microgravity means very small gravity. You must be very careful with this definition, though, because it refers to the overall effect of all the accelerations (acceleration is the term used to describe a change in motion) moving you, including gravity.
It is not called microgravity because the Earth's gravity is so small (remember, orbit is only possible because of gravity). Rather, it refers to the fact that you are freely falling toward the Earth along with all your surroundings, so the overall effect of gravity on you appears to be very small.
If you were an astronaut, you could let go of your pen and watch it gently float across the cabin. With a little push against the wall, you would glide through the air with the greatest of ease. You would be in the strange world of microgravity; but never fear, the Earth's gravity is still holding on to you quite tightly; slow down a little and you will be back on the surface. To help explain that gravity is present when the shuttle is in orbit, demonstrate Shuttle on a String.