Undergrad Does the Normal contact force act through the centre of mass?

[phantomvommand]

TL;DR

Does the Normal contact force always act through the centre of mass?

Consider the following situation:

You have 1 rectangular block lying on a table, and an identical block is placed above the block on the table. Now, this new block is constantly pushed to the right, right before it topples off.

Consider the torque about an axis passing through the rightmost and topmost point (this point is labelled A) of the bottom block. I read somewhere that the maximum this new block can be pushed is to the point where it Weight acts through A. Does this mean Normal contact force also only acts through A at this point?

Thank you!

 

[A.T.]

Does the Normal contact force always act through the centre of mass?

No. Normal contact force is a general concept that has nothing to do with the centre of mass per se.

Consider the following situation:

You have 1 rectangular block lying on a table, and an identical block is placed above the block on the table. Now, this new block is constantly pushed to the right, right before it topples off.

Consider the torque about an axis passing through the rightmost and topmost point (this point is labelled A) of the bottom block. I read somewhere that the maximum this new block can be pushed is to the point where it Weight acts through A. Does this mean Normal contact force also only acts through A at this point?

Yes, in this special case, where the normal forces must not create any net torque, their effective sum has to pass through the center of mass.

 

[phantomvommand]

No. Normal contact force is a general concept that has nothing to do with the centre of mass per se.Yes, in this special case, where the normal forces must not create any net torque, their effective sum has to pass through the center of mass.

I was really confused just now. Yes, it does not always go through the COM. Normal force is perpendicular to the contact surface, right?

In the situation I mentioned earlier, why is it that the maximum rightwards displacement is when Weight acts through the pivot point?

I suppose it is because when the Centre of Mass moves beyond the edge, normal contact force (acting perpendicular to the contact surface) can never balance the torque due to weight, resulting in rotation, loss of contact, and normal contact force = 0 eventually?

 

[A.T.]

Normal force is perpendicular to the contact surface, right?

Yes, that's all it means.

I suppose it is because when the Centre of Mass moves beyond the edge, normal contact force (acting perpendicular to the contact surface) can never balance the torque due to weight, resulting in rotation, loss of contact, and normal contact force = 0 eventually?

I would put it like this: Uniform gravity doesn't create any torque around the CoM, so the torque around the CoM by the supporting froces must also be zero to prevent rotation. With all the contact surface on one side of the CoM, there is no possible distribution of a non-zero support froce, that would create zero torque around the CoM.

 

[Lnewqban]

I was really confused just now. Yes, it does not always go through the COM. Normal force is perpendicular to the contact surface, right?

In the situation I mentioned earlier, why is it that the maximum rightwards displacement is when Weight acts through the pivot point?

I suppose it is because when the Centre of Mass moves beyond the edge, normal contact force (acting perpendicular to the contact surface) can never balance the torque due to weight, resulting in rotation, loss of contact, and normal contact force = 0 eventually?

Same thing will happen in the case the surfaces of contact between both blocks are not horizontal.
Normal forces will have one direction and gravity force will have another, yet the top block will start rotating around point A clockwisely as soon as it has been pushed towards the right enough as to have more mass to the right than to the left of an imaginary vertical line that intersects point A.