For the Love of Physics by Walter Lewin Page A

larger the frictional force. Thus, if the Rotor spins fast enough with the floor lowered, the frictional force can be large enough that it will balance the force of gravity and thus you won’t slide down.
    There are lots of ways to demonstrate artificial gravity. Here’s one you can try at home; well, in your backyard. Tie a rope to the handle of an empty paint can and fill the can with water—about half full, I’d say, otherwise it will be awfully heavy to spin—and then whip the can around as hard as you can up over your head in a circle. It might take some practice to get it going fast enough. Once you do, you’ll see that not a drop of water will fall out. I have students do this in my classes, and I must say it’s a complete riot! This little experiment also explains why, with some especially pernicious versions of the Rotor, it will gradually turn over until you are completely upside down at one point, and yet youdon’t drop down to the ground (of course, for safety’s sake, you are also strapped into the thing).
    The force with which a scale pushes on us determines what the scale tells us we weigh; it’s the force of gravity—not the lack of it—that makes astronauts weightless; and when an apple falls to Earth, the Earth falls to the apple. Newton’s laws are simple, far-reaching, profound, and utterly counterintuitive. In working out his famous laws, Sir Isaac Newton was contending with a truly mysterious universe, and we have all benefited enormously from his ability to unlock some of these mysteries and to make us see our world in a fundamentally new way.

CHAPTER 4

    The Magic of Drinking with a Straw
    O ne of my favorite in-class demonstrations involves two paint cans and a rifle. I fill one can to the rim with water and then bang the top on tightly. Then I fill the second can most of the way, but leaving an inch or so of space below the rim, and also seal that one. After placing them one in front of the other on a table, I walk over to a second table several yards away, on which rests a long white wooden box, clearly covering some kind of contraption. I lift up the box, revealing a rifle fastened onto a stand, pointing at the paint cans. The students’ eyes widen—am I going to fire a rifle in class?
    “If we were to shoot a bullet through these paint cans, what would happen?” I ask them. I don’t wait for answers. I bend down to check the rifle’s aim, usually fiddling with the bolt a little. This is good for building up tension. I blow some dust out of the chamber, slide a bullet in, and announce, “All right, there goes the bullet. Are we ready for this?” Then standing alongside the rifle, I put my finger on the trigger, count “Three, two, one”—and fire. One paint can’s top instantly pops way up into the air, while the other one stays put. Which can do you think loses its top?
    To know the answer, you first have to know that air is compressibleand water isn’t; air molecules can be squished closer in toward one another, as can the molecules of any gas, but those of water—and of any liquid at all—cannot. It takes horrendous forces and pressures to change the density of a liquid. Now, when the bullet enters the paint cans, it brings a great deal of pressure with it. In the can with the air in it, the air acts like a cushion, or a shock absorber, so the water isn’t disturbed and the can doesn’t explode. But in the can full of water, the water can’t compress. So the extra pressure the bullet introduces in the water exerts a good deal of force on the walls and on the top of the can and the top blows off. As you may imagine, it’s really very dramatic and my students are always quite shocked.
    Surrounded by Air Pressure
    I always have a lot of fun with pressure in my classes, and air pressure is particularly entertaining because so much is so counterintuitive about it. We don’t even realize we are experiencing air pressure until we actually look for it, and then it’s