- #1

pharcycle

- 5

- 0

Hello All,

I've been searching for days to try and solve this but I'm going round in circles so thought I'd fire it out to into the ether! I'm analysing a gantry system that is essentially cantilevered from one end but the part of the problem I'm struggling with can be simplified thus:

Imagine pushing a ruler on a desk from one end so that it has a tendency to want to rotate. To keep it moving as a pure translation you also apply a torque to the end. How do you work out what that torque it!?

I must be missing something brutally simple here but for the life of me I can't work it out. We can ignore friction for the purposes of this and since we're moving across a surface the force due to gravity is out of plane.

From the sum of forces = ma, it's pretty simple to work out what the applied force is to get it to accelerate. Summing the moments to zero (I'm assuming zero here as it's in pure translation) varies depending on where you take moments about so clearly something is wrong with my FBD here but I can't work out what. I don't think it should be sum of torques = I * alpha as it's not rotating, although I could certainly do that to get an answer.

It's doubly frustrating as 10 years ago I'd have been able to do this in a heartbeat but instead I've had to waste hours on this!

Hope someone can help and sorry my first post is a question!

Thanks,

David

I've been searching for days to try and solve this but I'm going round in circles so thought I'd fire it out to into the ether! I'm analysing a gantry system that is essentially cantilevered from one end but the part of the problem I'm struggling with can be simplified thus:

Imagine pushing a ruler on a desk from one end so that it has a tendency to want to rotate. To keep it moving as a pure translation you also apply a torque to the end. How do you work out what that torque it!?

I must be missing something brutally simple here but for the life of me I can't work it out. We can ignore friction for the purposes of this and since we're moving across a surface the force due to gravity is out of plane.

From the sum of forces = ma, it's pretty simple to work out what the applied force is to get it to accelerate. Summing the moments to zero (I'm assuming zero here as it's in pure translation) varies depending on where you take moments about so clearly something is wrong with my FBD here but I can't work out what. I don't think it should be sum of torques = I * alpha as it's not rotating, although I could certainly do that to get an answer.

It's doubly frustrating as 10 years ago I'd have been able to do this in a heartbeat but instead I've had to waste hours on this!

Hope someone can help and sorry my first post is a question!

Thanks,

David