At first , I shoul say : I am sorry , just now i posta a question , but my answer is aplly other question. I have edited it now. 1) If you were standing upright, facing forward but moving at a relativistic velocity to your right, which of the following quantities would change : height, width , pulse rate, number of atoms in my body, mass , temperature, rate of ageing, girth (waistline), or net electrical charge ? : I think the mass, width ,pulse rate and rate ageing are changed. rate ageing chage , I take the reason is twin paradox. 2) As the head traffic engineer for the department of highways, you are in charge of setting a safe but fuel-efficient speed limit on cars. Calculate a speed limit using relativistic equations and explain your reason for your choice of speed limit. Keep in mind that collisions cause changes in kinetic energy. : My idear is v=s/t, in the relativity, t own two value, one is relativety time, one is proper time, but the s also have two value, so how can i think this question ? Please give me some idea or remind. 3)Assume light speed is 30m/s in our world, Aliens visiting your planet from another quadrant of the galaxy, where the speed of light is 3*10^8 m/s,say your food seems relatively cold. Explain. : I think , because aliens world ,light speed is so fast, they look our world is slowly. as we cook hot food , because their time is fast, the food is cold. P.S. : last question , I lose a condition , is light speed is 30 m/s in our world. all of these question is in a relativity thought experiment . Please help me check these answer again. Thanks very much.