I am having trouble with volumes using the definite integral. I wanted to place a couple of questions from my textbook. I was wondering does anyone have any good tips. I have never been good at visualizing volumes even if I sketch the graph on the paper. If anyone could help me I would really appreciate it. Here are two examples from my textbook:

Region bounded by square root of x, y= 2 and x= 0 about (a) the y-axis; (b) x=1

y= e^x, x = 0, x = 2 and y = 0 about (a) the y-axis; (b) y = -2. Estimate numerically.

Thanks very much.