The diameter of a steel rod is 56.47 ± 0.02mm
What does it mean please

The rod is nominally 56.47mm in diameter but, due to manufacturing tolerances, there could be some variation.

The supplier is guaranteeing that the rod will not be smaller than 56.45mm nor will it be larger than 56.49mm in diameter.

Explanation:

It's means that the range of diameter of steel rod during its measurements will fall in between of 56.47–0.02mm= 56.45mm to 56.47+0.02mm= 56.49

+/- 0.02mm is the basically called as tolerance level.

Hope you get it.

r = ρ x length / area  

ρ = r x area / length  

ρ is in ohm-meters  

in basic units, it is m³kg/s³a²  

i think this is it.