Aperson yells at a canyon wall 200 m away. how long after they yell will they hear their echo

What property is a characteristic of a substance that can be observed and does not change the identity of the substance

The charge on an electron is 1.60218 × 10−19 c and its mass is 9.10939 × 10−31 kg. how strong a magnetic field must be experienced by the electron if its path is a circle of radius 5.5 cm? answer in units of t.

What does si stand for, when referring to a system of measurement