The two units are Inches of Mercury and Millibars
Metals contain the most and semiconductors contain the least amount. Hope this helps.
Answer:
Explanation:
Case-A
Block weight 
Friction Force 
and friction force is

where N=normal reaction (Usually equals to weight)


Case-B
Block weight 
Friction Force 



Case-C
Block weight 
Friction Force 



I think it is the first law of motion.