Answer from cs61c-ew (Joo-Rak Son 16103505) for Question 1
A virtual address is an address at a virtual memory, which represent and mange the two levels of the memory hierarchy in the main memory alone. A physical address is the real address at the main memory. To get from a va to a pa, one has to translate it using a combination of hardware and software. This can be done through memory mapping (or address translation) as the virtual addresses can be mapped to physical addresses.