In: Physics
2a) It is necessary to level the platform, In this experiment we are trying to measure centripetal force by rotation. Imagine If the rotating plane is tilted then the force of gravity which was acting perpendicular to the string is now at some angle, which has it's some contribution to the tension in string and hence your experiment setup will fail to measure the correct centripetal force due to rotation with some angular velocity.
2b) Similar to the previous problem, We can imagine a same kind of situation where at first string is parallel to the platform and has the tension equal to the centripetal force but if the string is making some angle with the platform then some component of tension will be in perpendicular direction and the net centripetal force applied will not be equal to the tension in the string, While for this case, (centripetal force = Tension horizontal in string) and for string parallel to the base ( centripetal force = Tension)