How does kinetic friction occur between two moving bodies?

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SUMMARY

Kinetic friction occurs when two surfaces in contact move relative to each other, with the force opposing this motion. The molecular interactions at the surface level, including irregularities that act like hooks, contribute to the resistance experienced. Kinetic friction always acts in the opposite direction of relative motion, as confirmed by multiple participants in the discussion. The relationship between friction and normal force is often treated as an engineering approximation rather than a fundamental physics principle.

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  • Understanding of basic physics concepts, particularly Newton's laws of motion.
  • Familiarity with the Coulomb model of friction.
  • Knowledge of surface interactions and material properties.
  • Basic grasp of relative motion and reference frames.
NEXT STEPS
  • Research the Coulomb model of friction and its applications in engineering.
  • Explore the molecular dynamics of surface interactions in materials science.
  • Study the effects of surface area on frictional forces in different contexts.
  • Learn about free body diagrams and their use in analyzing forces in motion.
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Physics students, engineers, and anyone interested in understanding the principles of friction and motion in mechanical systems.

  • #31
parshyaa said:
A body slipping on a rough horizontal plane moves with a deceleration of 4m/s^2.

In this case we are taking kinetic friction as the reason for accelaration
But kinetic friction resist the relative motion, then what is wrong, if in this case its helping to accelarating

You said opposite things. First you say it is decelerating (slowing down; eventually to stop). Then you say it accelerating (speeding up). You are confusing yourself.
 
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  • #32
parshyaa said:
In this case we are taking kinetic friction as the reason for accelaration
But kinetic friction resist the relative motion, then what is wrong, if in this case its helping to accelarating

Kinetic friction resists motion, that is correct.
The velocity is positive so Fkinetic friction is negative and so the acceleration is negative (=deceleration).

PS: Fkinetic friction is not a constant force. As soon as the velocity reaches zero Fkinetic friction becomes zero and is replaced by static friction. So Fkinetic friction cannot make the block accelerate in the opposite direction.
 
  • #33
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IMG_20171130_212328000_HDR.jpg

If this fbd is correct then fk is in the direction of accelaration this is why i said its towards acceleration or more precisely its the reason for accelaration due to fbd, but why kinetic friction is helping to acc. In this case
 

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  • #34
parshyaa said:
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View attachment 215847
If this fbd is correct then fk is in the direction of accelaration this is why i said its towards acceleration or more precisely its the reason for accelaration due to fbd, but why kinetic friction is helping to acc. In this case
The frictional force and the associated acceleration are both in the direction opposite to the relative motion. What is the problem?
 
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  • #35
jbriggs444 said:
The frictional force and the associated acceleration are both in the direction opposite to the relative motion. What is the problem?
Yes you are right, my confusion has gone now

I could solve problem containing two or three surfaces but solving questions containing only two surfaces sometimes makes me confuse but now i am cleared
Thanks @jbriggs444 and @anorlunda
 

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