# Rotating mass

imanator
Hi there, I'm new to the forum, but hopefully this question has a simply answer.

my question is, suppose you have a mass (a) on the end of a string or rod attached to a vertical support which is grounded. The mass is rotating about that support. Now assume you have another mass (b) traveling towards the rotating mass along a path, so that at one point, it is tangential to the radius of mass (a)'s rotation. when mass (b) reaches the rotating mass (a) and collides with it there will be an action reaction. my Question is will the reaction felt by mass (a) be through the centre support and in the same direction as mass (b) was going? or will it just induce a torque at the centre point, in other words if there was a force sensor at the centre point that was directed back (in the direction that mass (b) was coming from, would it register the same reaction force that object (b) felt when it hit object (a)? and what about the momentum of the system?

I hope this question is not too difficult to follow. I Attached a sketch for guidance.

I have done high-school physics and university physics, so I should be able to follow most replies.

Regards.

#### Attachments

• Rotating mass.jpg
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attachment is unreadable even at max magnification

imanator
Thanks for that, I uploaded a better one.

Mentor
A force sensor at the center won't feel the collision itself as there are no radial momentum changes. Afterwards, the changed rotation speed leads to a different radial force.

imanator
thanks mfb, not sure what you mean by afterwards the changed speed leads to different radial force though. are you referring to the fact the fact that the rotating mass has slowed down will result in a lower v^2/r value?

Mentor
thanks mfb, not sure what you mean by afterwards the changed speed leads to different radial force though. are you referring to the fact the fact that the rotating mass has slowed down will result in a lower v^2/r value?
Right.

imanator
does everyone else agree with this?