Mass of a particle depends on its speed. Does the attraction of the earth on the particle also depend on the particle’s speed?

A uniformly moving train passes by a long platform. Consider the events ‘engine crossing the beginning of the platform’ and ‘engine crossing the end of the platform’. Which frame (train frame or the platform frame) is the proper frame for the pair of events?

An object may be regarded to be at rest or in motion depending on the frame of reference chosen to view the object. Because of length contraction it would mean that the same rod may have two different lengths depending on the state of the observer. Is this true?

A person travelling in a fast spaceship measures the distance between the earth and the moon. Is it the same, smaller or larger than the value quoted in this book?

The magnitude of linear momentum of a particle moving at a relativistic speed v is proportional to