Chapter 4: Fundamental Interactions

(Press the TAB key to position the cursor at the beginning of the first blank and type what you believe completes the thought. Then press the TAB key for feedback and to move to the next blank. If you miss it, delete all of the word by positioning the cursor with the mouse and backspacing over all of the letters and try again. You should be able to complete this exercise without error before completing this chapter.)

There are only four fundamental forces: strong, electromagnetic, weak, and gravity. Of these, the electromagnetic and gravititational forces are responsible for most of what we see with our naked eyes. Since all forces arise in interactions between two objects, we sometimes say that there are only four fundamental interactions. We first consider gravity. Every object near the surface of the earth experiences a downward force (called its ), the strength of which is exactly proportional to its . If we hold the mass of an object constant, but significantly increase the distance between the object and the center of the earth, the acceleration due to gravity of the object (increases, decreases?) . We therefore conclude that the attraction of the earth for the object (does, does not?) depend on the distance between their centers. In fact, the force decreases (inversely) as the of the distance. If the distance between two objects were doubled, the resulting force of attraction between the objects would be one (half, third, fourth?) of its previous strength. The formula to help remember the things that the gravitational force depends on is: Fg= GmM/d2. Under comparable circumstances, the electromagnetic interaction is (stronger, weaker?) than the gravitational interaction. Charged objects that exert forces on one another obey Newton's Law. The strength of the force depends on the (masses, charges?) , q and Q, of the two objects, and the distance between them according to: Fe= kqQ/d2. Unlike the gravitational interaction, the electromagnetic interaction may be attractive or repulsive. The strength of the electrical forces (does, does not?) depend on the masses of the two objects. Electrical Model of Matter: 1) Matter contains two kinds of charges, positive protons and negative . (Electrons, protons?) can easily be transferred from one object to another. 2) Objects with equal numbers of protons and electrons are electrically . 3) Objects with more electrons than protons are . Objects with fewer electrons than protons are .

Click here to return to the Table of Contents