I Newton's second law for rotations

[pedrovisk]

TL;DR Summary

What is the minimum force to start rotation in a bar?

EDIT: I forgot about Second Newton's law for rotations and this led to a mistake. Anyway, thanks for the people who answered it and remembered me about law of inertia.

I was thinking about how to "make" things to move without rotate the object, then i tried to calculate the minimum force to start a homogeneous bar to rotate in a x axis. I attached a link.
Could someone check if my answer is correct?

 

[PeroK]

What force is resisting rotation?

 

[pedrovisk]

What force is resisting rotation?

Hi! The bar is at static equilibrium. What I meant with this force F is what "push" I need to do in order to "win" the moment of inertia.

 

[PeroK]

Hi! The bar is at static equilibrium. What I meant with this force F is what "push" I need to do in order to "win" the moment of inertia.

I don't know what "win" means in this context. In the absence of a resisting force, any force (no matter how small) will move and rotate the bar.

 

[Orodruin]

It is like asking what force is required to make an object accelerate when there is no resisting force. F = ma tells you that any net force will result in an acceleration, just as $T = I\alpha$ tells you any net torque will result in an angular acceleration.

 

[pedrovisk]

It is like asking what force is required to make an object accelerate when there is no resisting force. F = ma tells you that any net force will result in an acceleration, just as $T = I\alpha$ tells you any net torque will result in an angular acceleration.

Can't believe i forgot about this. Basic Newton's first law for rotations.

I think I was confused when I was trying to rotate a bar like that. I probably just forgot about the friction.

Anyway, now I'm curious. Is there any meaning for the pic I attached?

 

[pedrovisk]

I don't know what "win" means in this context. In the absence of a resisting force, any force (no matter how small) will move and rotate the bar.

Thanks, you are absolutely right. I was having trouble to understand rotations as a extension of Newton's laws.

Asking the same question I did to Orodruin, what is a possible interpretation for the pic? Did it have a meaning or I should just torn it apart the paper and throw it to the trash? The development of the equations looks so smooth.

 

[PeroK]

Thanks, you are absolutely right. I was having trouble to understand rotations as a extension of Newton's laws.

Asking the same question I did to Orodruin, what is a possible interpretation for the pic? Did it have a meaning or I should just torn it apart the paper and throw it to the trash? The development of the equations looks so smooth.

You have a torque of $F\frac l 2$. Somehow you have an opposing torque of $\frac{mlg}{4}$.

 

[PeroK]

PS gravity acts through the centre of the bar, so it should not produce a torque on the bar about its centre.

 

[A.T.]

I was thinking about how to "make" things to move without rotate the object,

Apply all force through the center of mass, otherwise make sure that the torques cancel.