The speed of an object is not a property of the object alone: it is a property of the object in relation to another object. The speed of a child on a moving train has a value relative to the train (a few steps per second) and a different value relative to the ground (a hundred kilometers per hour). If his mother tells the child to 'Keep still!', she does not mean that they have to throw themselves out of the window to stop in relation to the ground. She means that the child should stop with regard to the train.
Tags: science, physics