I am reading Michio Kaku books and Brian Green Books and I am planing reading Stephen Hawking books and I have a couple of question please give me a link if it already have been answered. 1-If I move at 90% speed of light and trow something at 90% speed of light in the same direction that object should move approximately at 99% speed of light not 180%. Now my question is 2 object or person moving at 90% speed of light toward each other at witch speed will they see each other approaching (99% I think)? Also does an outside observer see them closing each other at 180% the speed of light ? Also if I am right and the outside observer see them at 180% but they see themselves going at 99% if they hit each other at witch speed would you calculate the impact collision to determine the trajectory and speed of the debris from the impact? 2-I read that mass of the object increase with speed and near speed of light object reach near infinite mass that's why you cant pass the speed of light because it would be infinite that's why it's impossible according to Einstein. But I think the mass increase is relative right ? I mean the person or object does not see itself with a mass increase its just the observer. My question is what happen when you spin on yourself like tennis ball are you heavier too? If the ball rotate in perfect topspin from the point of view of the ball say bottom, top or side of the ball it may appear that you are in perfect no motion state and it is the world that is spinning around the ball so no mass increase from that point of view. But when the ball hit the racket the ball would be heavier say then a flat ball with no spin ? I understand that a tennis ball may not spin fast enough to feel the difference but every tennis player feel topspin ball like if they are much heavier when they hit the racket so I was wondering why ?