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Trouble Understanding Friction?

Shuddhodan Sapre
15 Apr 0 0

Many of my students make a widespread mistake of taking friction value as f= μ N. Where μ = coefficient of friction and N =Normal Reaction.

So is it incorrect? The answer is not exactly. With that, I mean to say that the above formula gives you maximum friction.

Still, confused? Read below

Consider a block on a rough surface with μ= 0.5. Now let's say this block has a mass of 10kg, and you are pushing the block in the horizontal direction towards the right with a force of 150N. If we take acceleration due to gravity g as 10m/s², what we can calculate is as follows

The Normal Reaction on the block will be

N=mg = 10 x10= 100N.

So the Max Friction Value here is

f= μ N = 0.5 x 100 = 50N

The block is acted upon by two forces, one on the right, which you applied (150N) and max friction on the left (50N), so the block moves rightwards with 150 - 50 = 100N.

But!!!!!!!!!!!!!!

Consider the same scenario, instead of pushing with 150N, what if you would have pushed with, say, a force of 30N.

If you take the friction value as 

f= μ N = 0.5 x 100 = 50N

With that logic, you pushed the block rightwards with 30N and friction acting on the block leftwards with 50N, so basically, the block moves towards the left with 50-30 =20N.

Common! Think is it possible?? You are pushing something on the right, and it moves on the left??

So what mistake are we making?

See the frictional force which u take as f= μ N is maximum, and the friction force can take any value between 0 and f= μ N. So when you apply a force less than f= μ N, friction force will be the same value as that force if no other forces are acting.

In the above example, when you apply 30N, the friction acting is also 30N; this cancels the external force, and the block remains Stationary.

Now that makes sense.

Cheers!

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