For the Love of Physics by Walter Lewin

lying down than when you’re standing up. Well, when she made a salad, she really had a good time. She would wash the lettuce in a colander, and then rather than drying it in a cloth towel, which would damage the leaves, she had invented her own technique: she took the colander and put a dish towel over the top, holding it in place with a rubber band, and then she would swing it around furiously in a circle—I mean really fast.
    That’s why when I demonstrate this in class, I make sure to tell the students in the first two rows to close their notebooks so their pages don’t get wet. I bring lettuce into the classroom, wash it carefully in the sink on my table, prepare it in the colander. “Get ready,” I tell them, and I swing my arm vigorously in a vertical circle. Water drops spray everywhere! Now, of course, we have boring plastic salad spinners to substitute for my grandmother’s method—a real pity in my book. So much of modern life seems to take the romance out of things.
    This same artificial gravity is experienced by astronauts as they accelerate into orbit around the Earth. A friend and MIT colleague of mine, Jeffrey Hoffman, has flown five missions in the space shuttle, and he tells me that the crew experiences a range of different accelerations in the course of a launch, from about 0.5 g initially, building to about 2.5 g at the end of the solid fuel stage. Then it drops back down to about 1 g briefly, at which point the liquid fuel starts burning, and acceleration builds back up to 3 g for the last minute of the launch—which takes about eight and a half minutes total to obtain a speed of about 17,000 miles per hour. And it’s not at all comfortable. When they finally reach orbit they become weightless and they perceive this as zero gravity.
    As you now know, both the lettuce, feeling the colander pushing against it, and the astronauts, feeling the seats pushing against them, are experiencing a kind of artificial gravity. My grandmother’s contraption—and our salad spinners—are of course versions of a centrifuge, separating the lettuce from the water clinging to its leaves, which shoots out through the colander’s holes. You don’t have to be an astronaut to experience this perceived gravity. Think of the fiendish ride at amusement parks called the Rotor, in which you stand at the edge of a large rotating turntable with your back against a metal fence. As it starts to rotate faster and faster, you feel more and more pushed into the fence, right? According to Newton’s third law, you push on the wall with the same force as the wall pushes on you.
    This force with which the wall pushes on you is called the centripetalforce. It provides the necessary acceleration for you to go around; the faster you go, the larger is the centripetal force. Remember, if you go around in a circle, a force (and therefore an acceleration) is required even if the speed remains unchanged. In similar fashion, gravity provides the centripetal force on planets to go around the Sun, as I discuss in appendix 2 . The force with which you push on the wall is often called the centrifugal force. The centripetal force and the centrifugal force have the same magnitude but in opposite direction. Do not confuse the two. It’s only the centripetal force that acts on you (not the centrifugal force), and it is only the centrifugal force that acts on the wall (not the centripetal force).
    Some Rotors can go so fast that they can lower the floor on which you stand and you won’t slide down. Why won’t you slide down?
    Think about it. If the Rotor isn’t spinning at all the force of gravity on you will make you slide down as the frictional force between you and the wall (which will be upward) is not large enough to balance the force of gravity. However, the frictional force, with the floor lowered, will be higher when the Rotor spins, as it depends on the centripetal force. The larger the centripetal force (with the floor lowered), the