A) https://www.youtube.com/watch?v=wiBsfvW5AWY
B) https://www.youtube.com/watch?v=i88ipC8GebA
And just for good measure on a more intuitive understanding of special relativity, https://www.youtube.com/watch?v=Vitf8YaVXhc and https://www.youtube.com/watch?v=Zkv8sW6y3sY
I love this guy's videos; the best I have ever seen difficult physics concepts explained, based on intuition and not equations.