How is the production of mercury from burning coal an environmental hazard?
a.
High concentrations of mercury in the air leads to global warming.
b.
Mercury in water systems is transformed through biological processes into a substance that is toxic to animals.
c.
Mercury combines with other coal emissions in the air to produce ozone.
d.
All of the above

It depends on the surface of the levels. the rougher the surface, the greater is the frictional force. for rougher surfaces, you would need to exert greater force to move the object. the concept is that, you must overcome the frictional force in order to bring the object into motion. this is the reason why we add lubricants to the joints so they could easily move. this answer was on

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