Friction and Moments

Hi,
Can anyone help me with understanding the following concept about friction in context of moments? The thing I can't seem to get is where exactly does friction acts (i.e. the point at which friction acts) and where does normal reaction acts (i.e. the point at which normal reaction acts) as it is really important while we are evaluating moments. Like in the case of a Box moving on ground, I have attached the diagram.

Friction:
Friction force always acts tangent to the surface at points of contact.

Normal Reaction Force:
Normal Reaction force acts in a direction that is perpendicular to the surface of contact.

Friction.jpg159.8KB

Reply 1

mqb2766

Original post by Tesla3

Friction acts against movement (or attempted movement). So it would act horizontally in the opposite direction to the movement, or pointing left in your diagram.
Reaction is the surface pushing back vertically against weight as the body does not move vertically. Weight acts downwards, the normal reaction fotce acts upwards.

(edited 4 years ago)

Reply 2

Tesla3

Original post by mqb2766

No you are not getting what I am trying to say, I am precisely interested at which point these two forces are acting as in evaluating moments we need the exact point at which the forces are acting so we can evaluate the distance from the pivot. Like for example, the weight acts at centre of mass of a body.

Reply 3

mqb2766

Original post by Tesla3

The normaL reaction force would act under the COM. Where, it depends on the size of the base and would usually be evenly distributed across it. Friction acts at the base, but the size of the object is generally neglected, so any moment due to the force not applying through the com is generally neglected at this level. I dont know what you want to assume. The com isnt even on the diagram.
If someone is trying to help you, chill a bit?

(edited 4 years ago)

Reply 4

Tesla3

Original post by mqb2766

Will it be something like this.....?

(edited 4 years ago)

Friction1.jpg223.2KB

Reply 5

mqb2766

The simple diagram is
https://en.m.wikipedia.org/wiki/Normal_force
As long as the weight acts through the base, no moment due to the weight occurs. You can imagine the normal reaction force acting through the com, but really it occurs at all points where the base makes contact with the surface.
In the same way, friction acts at the base of the object, at the point of contact between the base and the surface. It would occur at all points of contact on the base. Again, the moment due to friction not acting through the com is neglected.
Try putting them on your diagram?

Reply 6

Tesla3

Original post by mqb2766

Sorry, I hope I did nothing wrong to upset you. Clearly, I was going too fast. I have drawn the diagram, will it be something like this?....

(edited 4 years ago)

Friction2.jpg252.0KB

Reply 7

mqb2766

Original post by Tesla3

Sorry, I hope I did nothing wrong to upset you. I was clearly going too fast. I have drawn the diagram, will it be something like this?....

Friction would act at the base of the object and is often drawn at the "front", relative to the motion. The moment due to friction acting under the com is generally neglected / insignificant. Yours is ok, but the usual convention is front / base.
Also, the weight / reaction is correct. If the box just had two pounts of contact at the ends of the box (like a suspended rod), the reaction force would act at those points.. Generally its just drawn as a force acting through the com and applied at the point of base/surface contact which goes through the com.

(edited 4 years ago)

Reply 8

mqb2766

Original post by Tesla3

Umm, in 3d, the surface between the box and the ground will be a plane having an equation of the form ax+by+cz=d. Lets say that this plane is of a finite size according to the side length of the box. As we already said that frictional force always acts tangent to the surface at points of contact. Therefore, tangent to any point on the plane will tell us the direction of frictional force at that point. Am I right?

Sounds about right, but i dont understand your diagram.
Just imagine the surface and the box's base being two sheets of horizontal sandpaper on a table., You push down a bit on the top sandpaper, but try and slide the top sheet horizontally. Friction (a horizontal force) occurs to restrict horizontal motion because of all the tiny, vertical "teeth" on the sheets of sandpaper. It is simply modelled as a horizontal (along the surface) force acting at the point of contact between the sheets.
For just about all these box problems, you can neglect the box's size as this has been abstracted out of the problem.

(edited 4 years ago)

Reply 9

Tesla3

Sorry for asking an amateur question, but I can't really understand the definition of friction that I read online.
(i.e. Friction force always acts tangent to the surface at points of contact.)

What does " tangent to the surface at points of contact mean " ? My understanding tells me that tangent always passes through one point but why is it saying that tangent passes through points of contact? Can you explain it in layman terms?.....@mqb2766

(edited 4 years ago)

Reply 10

mqb2766

Original post by Tesla3

Sorry about asking an amateur question, but I can't really understand the definition of friction that I read online.
(i.e. Friction force always acts tangent to the surface at points of contact.)

What does " tangent to the surface at points of contact mean " ? My understanding tells me that tangent always passes through one point but why is it saying that tangent passes through points of contact? Can you explain it in layman terms?.....@mqb2766

Its correct. Tangent means along the surface. When the surface is curved, it will be along the gradient (tangent) at the point of contact,
Imagine balancing a book on a fixed, statiknary ball. The reaction would be normal to the surface of the book at the point of contact with the ball and friction would occur along (in the direction of the plane of) the book (tangent) if you tried to slide the book along the surface of the ball.