I have a problem in mechanical physics where I'm given the linear mass density of a rigid plastic material. Here's the problem for context: A 42 cm diameter wheel, consisting of a rim and six spokes, is constructed from a thin, rigid plastic material having a linear mass denisty of 25 g/cm. The wheel is released from rest at the top of a hill 58 m high. Find the velocity at the bottom of the hill.
What is linear mass density??? I noticed in another problem it is equal to 2piRlambda in some cases, while only lambdaR in others. Where do these equations even come from??