A brownish-black sedimentary rock is known as (a) charcoal (b) coke (c) coal (d) coal tar

Do the structure of spherical mirror show pole centre of curvature radius of curvature principal Axis principle focus focal length of convex mirror

Trignometry rules about how the are being applied and the understandability to use them in every condition

Difference between ions, radicals and valency ? And naming method of variable vapency

What is the difference between vIon vs radical vs valency

I want  complete notes on the chapter friction 

In my book subtopics are:

Friction 

Types of friction 

Factors affecting friction 

Methods of reducing friction 

Methods of increasing friction 

Advantages and disadvantages of friction 

Some real life based situation where friction is applied.