How does kinetic friction occur between two moving bodies?

[anorlunda]

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.

 

[CWatters]

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 F_{kinetic friction} is negative and so the acceleration is negative (=deceleration).

PS: F_{kinetic friction} is not a constant force. As soon as the velocity reaches zero F_{kinetic friction} becomes zero and is replaced by static friction. So F_{kinetic friction} cannot make the block accelerate in the opposite direction.

 

[parshyaa]

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

 

[jbriggs444]

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?

 

[parshyaa]

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