If there is to be no relative motion between the train and the edge of the platform, how fast must the train move compared to the rim of speed of the rotating platform? (A please)

Train must move with equal speed as of platform in opposite direction i order to maintain zero relative velocity.

Explanation:

From the question it is given that

relative velocity between platform and train = 0

since relative velocity of train with respect to platform is given by

where, is the velocity of train and is the velocity of platform

⇒

since platform is always at stationary i.e.,

thus train velocity = 0

Therefore, train does not move

If, Platform is moving then train must move with equal speed as of platform in opposite direction

the body's situation won't change the forces are equal opposit and on the same body

hypothesising or postulating