Summary: ASTR 1120 Section 1 (3 credit hours): Spring 2006
SUMMARY OF KEY CONCEPTS: WEEK #2
Lecture #3 textbook 4.3 / 5.3 (4th
edition) or 4.2 / 4.3 (3rd
We discussed the concept of forces a force being something that changes the velocity of an object.
The four fundamental forces are gravity, electromagnetism, the weak nuclear force, and the
strong nuclear force. We noted that gravity is intrinsically the weakest it dominates over large
scales in the Universe because (a) electric charges come in positive and negative flavors, which
tend to cancel out, and (b) because the nuclear forces are very short range.
We then reviewed the structure of matter, which is made up of atoms themselves composed of a
nucleus (containing protons and neutrons) and electrons. The nucleus is small (10-15
contains almost all the mass, while the electrons orbit further out (10-10
m) and occupy most of
the volume. Chemical reactions involve changes in the distribution of electrons, whereas
nuclear reactions involve the nucleus.
We noted that a chemical element is defined by the number of protons in the nucleus (for a
neutral atom, this equals the number of electrons). Different isotopes of an element differ in
having different numbers of neutrons in their nuclei e.g. most carbon nuclei have 6 protons