Is a bike standing still potential or kinetic?

The speed of light in a material is 0.50 c. what is the critical angle of a light ray at the interface between the material and a vacuum?

Does anyone know how to solve this problem? i really need . i made an attempt but i just cant get it. a metal rod is 25.000 cm long at 25.0 degrees celsius. when heated to 102.0 degrees celsius, it is 25.054 cm long. what is the coefficient of linear expansion for this metal.

Using a caesar cypher with an offset of three characters (a -> d, b -> e, z -> c), what would be the correct cypher text for the plain text "attack at dawn"?