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

In summary, when the COM is located beyond the edge of a block, normal contact force (acting perpendicular to the contact surface) cannot balance the torque due to weight, and this will result in rotation, loss of contact, and normal contact force = 0 eventually.
  • #1
phantomvommand
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TL;DR Summary
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!
 
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  • #2
phantomvommand said:
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.

phantomvommand said:
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.
 
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  • #3
A.T. said:
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?
 
  • #4
phantomvommand said:
Normal force is perpendicular to the contact surface, right?
Yes, that's all it means.

phantomvommand said:
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.
 
  • #5
phantomvommand said:
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.
 

1. What is the normal contact force?

The normal contact force is the force that occurs when two objects are in contact with each other, pushing against each other perpendicular to the surface of contact.

2. Does the normal contact force always act through the centre of mass?

No, the normal contact force does not always act through the centre of mass. It depends on the orientation and shape of the objects in contact.

3. Why is it important to know if the normal contact force acts through the centre of mass?

It is important to know if the normal contact force acts through the centre of mass because it affects the stability and equilibrium of the objects in contact. If the force does not act through the centre of mass, it can cause the object to rotate or topple over.

4. How can I determine if the normal contact force acts through the centre of mass?

You can determine if the normal contact force acts through the centre of mass by analyzing the geometry and distribution of mass of the objects in contact. If the objects are symmetrical and evenly distributed, the force is likely to act through the centre of mass.

5. What happens if the normal contact force does not act through the centre of mass?

If the normal contact force does not act through the centre of mass, it can cause the object to rotate or topple over. This can lead to instability and potential damage to the objects or surrounding environment.

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