Physics

You, travelling in a train at half the speed of light, and a friend, standing on the platform, both make measurements of your surroundings. (Like Einstein, we will blithely pass over how the train came to be travelling at this speed.)a  You both decide to measure the length of the platform. What is the easiest way for your friend to do this? How can you do this?b  If your friend measures the length of the platform to be 20 m, what do you measure it to be? Which one of you is measuring a proper length?c  A previously unknown species of bird flies by you (in the same direction as the train is moving). If your friend measures its speed to be 0.75c, what do you measure its speed to be?