What is the difference between work done by and against friction?

Question

anonymous · Answer

when you are walking, you feet are working against friction. they slide back against the earth's surface hence providing you the push to move ahead. 

when your car tyre is rolling forward on the road, thats because friction is present between the tyre and the road to push it ahead. otherwise your tyre would keep slipping at the same place. like on ice.

anonymous · Answer

2nd example is the case of work done by friction.

plainious · Answer

So what your trying to say is that work done by friction is when you are looking at the work done in the direction of the frictional force (i.e opposing motion) whereas, work done against friction will be the work done in the direction of motion in overcoming the friction. I'm I right?

plainious · Answer

Please can you explain it a bit further (feel so dumb!), I'm still confused.

anonymous · Answer

I am sorry, may be I confused you with my reply earlier. I should have been more clearer.
These 2 statements - 

Work Done by a Force 

Work Done against a Force

Now lets assume, we have a system and a force is acting on it. 
for example, you can imagine yourself walking on a path in the woods (make the scenario as beautiful as you can).  
here, you are the system. and friction is the force acting on you. 
for simplicity, we will just assume that you are dragging your feet along, effectively just rubbing against the ground.

Lets look at the 2 statements one by one.


1). Work done by a Force 
Here, we are asking what is the energy spent by the force.  so now take your beautiful scenario of you walking in the woods. the friction is acting on the system against the system's direction of motion. 
what is the work done by friction ? its just the work done by friction to resist your motion.

2).
Work done against a force.
We are asking, what is the energy spent against a force, BY THE SYSTEM.  so when the system is moving forwards, how much energy is wasted in resisting the force of friction.

so you see,  Work done by friction force =  Work done BY THE SYSTEM AGAINST the friction force.  These 2 are equal. its just that in L.H.S, we are looking at friction force and in R.H.S, we are looking at the SYSTEM.

plainious · Answer

Thanks a lot! I get it now! That was very intuitive!

anonymous · Answer

you're welcome. Thanks for asking