Flight Safety,. .. .The drum is applying a force to the pads. That force causes the pads to accelerate towards the center of rotation.. .. .If you throw a baseball, prior to it leaving your hand there is friction between the baseball and your hand that prevents it from slipping (not rolling) across your palm. Your hand is pushing on the ball because you created a force that is transmitted through your hand. The ball does not create a force. Yes, you feel the reaction on your hand, but that isn't a 'real' force. In analogy, the drum is your hand the pad is the baseball.. .____________. .. .Lu, . .. .You're getting there.. .. .______________. .. .Dave,. .. .Push pull doesn't matter. They are both inwards. They are both centripetal.. .. .Whether the drum has mass or not is irrelevant because we aren't considering this as being accelerated inwards (we can...but we're not), just the pads. In reality the drum has mass. If it didn't it wouldn't be there. It is not always valid to extend physics to a singularity.