Free For the Love of Physics by Walter Lewin Page B

mission, David Scott, famously dropped a hammer and a falcon feather onto the surface of the Moon at the same time to see if objects of different mass would fall to the ground at the same rate in a vacuum. It’s a wonderful video, which you can access here: http://video.google.com/videoplay?docid=6926891572259784994# .
    The striking thing to me about this video is just how slowly they both drop. Without thinking about it, you might expect them both to drop quickly, at least surely the hammer. But they both fall slowly because the gravitational acceleration on the Moon is about six times less than it is on Earth.
    Why was Galileo right that two objects of different mass would land at the same time? The reason is that the gravitational acceleration is the same for all objects. According to F = ma , the larger the mass, the larger the gravitational force, but the acceleration is the same for all objects. Thus they reach the ground with the same speed. Of course, the object with the larger mass will have more energy and will therefore have a greater impact.
    Now it’s important to note here that the feather and the hammer would not land at the same time if you performed this experiment on Earth. This is the result of air drag, which we’ve discounted until now. Air drag is a force that opposes the motion of moving objects. Also wind would have much more effect on the feather than on the hammer.
    This brings us to a very important feature of the second law. The word net in the equation as given above is vital, as nearly always in nature more than one force is acting on an object; all have to be taken into account. This means that the forces have to be added. Now, it’s not really as simple as this, because forces are what we call vectors, meaning that they have a magnitude as well as a direction, which means that you cannot really make a calculation like 2 + 3 = 5 for determining the net force. Suppose only two forces act on a mass of 4 kilograms; one force of 3 newtons is pointing upward, and another of 2 newtons is pointing downward. The sum of these two forces is then 1 newton in the upward direction and, according to Newton’s second law, the object will be accelerated upward with an acceleration of 0.25 meters per second per second.
    The sum of two forces can even be zero. If I place an object of mass m on my table, according to Newton’s second law, the gravitational force on the object is then mg (mass × gravitational acceleration) newtons in the downward direction. Since the object is not being accelerated, the net force on the object must be zero. That means that there must be another force of mg newtons upward. That is the force with which the table pushes upward on the object. A force of mg down and one of mg up add up to a force of zero!
    This brings us to Newton’s third law: “To every action there is alwaysan equal and opposite reaction.” This means that the force that two objects exert on each other are always equal and are directed in opposite directions. As I like to put it, action equals minus reaction, or, as it’s known more popularly, “For every action there is an equal and opposite reaction.”
    Some of the implications of this law are intuitive: a rifle recoils backward against your shoulder when it fires. But consider also that when you push against a wall, it pushes back on you in the opposite direction with the exact same force. The strawberry shortcake you had for your birthday pushed down on the cake plate, which pushed right back at it with an equal amount of force. In fact, odd as the third law is, we are completely surrounded by examples of it in action.
    Have you ever turned on the faucet connected to a hose lying on the ground and seen the hose snake all over the place, maybe spraying your little brother if you were lucky? Why does that happen? Because as the water is pushed out of the hose, it also pushes back on the hose, and the result is that the hose is whipped all around. Or