Why are lubricants used in machines ?

Lubricant is used in machine to

Lubricant oil used in machines should be of high vescosity.Why?

Why are metals used for making wires ?

Why are two propellers used in a helicopter?

Show that the frequency revolution of a charged pawrticle in a uniform magnetic field vecB(vecB=Bhatk) , is independent of the speed. Which particle machine makes use of this fact? What is the frequency of the alternating electric field, used in this machine?

Why sky waves are not used in transmission of TV signals?

Why mercury is used in anaerobic respiration experiments?