Conceptual Problem with Friction FBD.

Click For Summary

Discussion Overview

The discussion revolves around a conceptual problem involving two blocks, A and B, where block B is placed on top of block A. The scenario explores the effects of friction between the two blocks when a force is applied to block A. Participants are examining the free body diagrams (FBDs) for both blocks and the implications of friction in this context.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant describes a scenario with two blocks, A and B, where block A is on a frictionless surface and block B is on top of A, with a coefficient of friction 'u' between them. They express confusion about the free body diagrams (FBDs) resulting in an apparent contradiction.
  • Another participant notes that the limiting friction is not necessarily equal to the actual friction force when a force equal to 'f' is applied to block A.
  • A subsequent reply suggests that if block A is pushed with a force equal to μmg, the friction acting between the blocks will be only half that, indicating a potential misunderstanding of the relationship between applied force and friction.
  • One participant expresses uncertainty about the previous statements and asks for clarification on how to determine the acceleration of the two-block system under the applied force.

Areas of Agreement / Disagreement

Participants do not appear to reach consensus on the nature of the frictional forces at play or the implications of the applied force on the system. There are competing views regarding the relationship between limiting friction and actual friction, as well as the resulting accelerations of the blocks.

Contextual Notes

There are unresolved assumptions regarding the initial conditions of the blocks, the nature of the applied force, and the definitions of limiting versus actual friction. The discussion reflects a complex interplay of forces that may not be fully articulated.

Sarin
Messages
8
Reaction score
0
here is the scenario:

there are 2 blocks A & B, both have the same mass 'm'.
block A is kept on an endless surface.
there is no friction between the block A and the surface.
now the other block, B, is kept on top of the block A.
the coeff of friction between A & B is say 'u'.
the limiting friction between A & B is therefore = umg = 'f'.
now say i apply a force 'F', equal to 'f', on the block A, i.e F = f.

What will be the FBD's for A and B.

I did the FBD for B & A and i am getting f = 0 = F , which is not possible since f = umg.
So I think i have made some mistake.

And I am not saying that A and B are necessarily stationary initially, they could be moving with a uniform velocity.

Is this suggesting that since there is no relative motion between A & B therefore there is no friction existing. If this is the case then isn't this contradicting the fact that B moves with A
(with a relative velocity = 0) due to friction when a force is applied to A and that if friction was not present A would just slide past underneath B and B would just fall to the ground.
kindly help me out with this!

Thanks.
 
Last edited:
Physics news on Phys.org
Sarin said:
the limiting friction between A & B is therefore = um = 'f'.
now say i apply a force 'F', equal to 'f', on the block A, i.e F = f.
Realize that the limiting friction is not necessarily equal to the actual friction force.

If you push A with a force equal to μmg, then the friction acting between the masses will be only half that.
 
Doc Al said:
Realize that the limiting friction is not necessarily equal to the actual friction force.

If you push A with a force equal to μmg, then the friction acting between the masses will be only half that.

not sure i understand what you're saying.
 
Sarin said:
not sure i understand what you're saying.
Try this: Assume that you push block A with a force equal to μmg. What will be the acceleration of the two-block system? Use that to figure out the friction force.
 

Similar threads

Undergrad Modeling Coulomb Friction for a Moving Point Mass in a Circular Bowl
  • · Replies 20 ·
Replies
20
Views
2K
Undergrad Solving Wheel Coming to a Stop: FBD, Friction & Hysteresis
  • · Replies 22 ·
Replies
22
Views
5K
Undergrad Reaction force of and its relation to normal force and friction
  • · Replies 20 ·
Replies
20
Views
5K
High School Deriving v(t) from F(x), for Linear Motion
  • · Replies 1 ·
Replies
1
Views
1K
Undergrad Rolling Without Slipping
  • · Replies 59 ·
2
Replies
59
Views
5K
Undergrad If frictional force in system is <= us*N then there is no slipping
  • · Replies 7 ·
Replies
7
Views
2K
Undergrad Determine if the block will slide or flip over after contact
  • · Replies 8 ·
Replies
8
Views
2K
High School Work and energy: conceptual doubt
  • · Replies 2 ·
Replies
2
Views
2K
High School Box being pulled up a slope with friction
  • · Replies 3 ·
Replies
3
Views
2K
Graduate Rack of blocks held horizontally under pressure
  • · Replies 52 ·
2
Replies
52
Views
4K