# Simulating opposing rotational forces at different points on structure

1. Jul 15, 2013

### AshSechler

Hi,

I am trying to simulate a tangential force acting on a point on a structure and the corresponding opposing force caused by the moment of inertia of a mass at a different position.

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In this image the structure rotates around point c and there is a vector out of mass m opposing the rotation caused by tangential force F.

My main question: What would the proper magnitude of the opposing force caused by the moment of inertia be?

If it is much easier to pretend that the structure is rigid, what kind of equation would I use? I have been able to find the angular acceleration based on the total torque and total moment of inertia. Is mass x radius x acceleration the right idea?

thanks!

Ash

Last edited by a moderator: May 6, 2017
2. Jul 16, 2013

### Simon Bridge

Welcome to PF;
Your diagram is incorrect - remove the arrow from the mass (unless you are also applying a force there.)
Model flexible rods/structures as a mass and spring system, then you can draw free-body diagrams as usual.

3. Jul 16, 2013

### AshSechler

Hey Simon,

thanks for your response. I should be more clear - the arrow out of the mass represents the reaction force, which i do need to manually add in this simulation.

4. Jul 16, 2013

### AshSechler

can anybody help me?

5. Jul 17, 2013

### Simon Bridge

- why would there be a reaction force at the mass like that?
- if it is mandated by your simulation, then your simulation does not appear to conform to known physics. The way you get such a force is if there is some sort of drag or friction there. Is this a program you have developed yourself or one you have been given or purchased?

6. Jul 19, 2013

### AshSechler

because of the limitations of the simulator, I must describe the effect of the mass(slowing the rate of acceleration) as a negative force as a function of the input force.

7. Jul 19, 2013

### Simon Bridge

Then your simulation does not conform to known physics and will give erroneous results.
You would basically have to calculate the effect of the inertia by hand, then work out what sort of "simulated force" would be needed to get the simulator to do the same thing - why not just write an animation and cut out the middle-man?

This is not physics.