What is the diffrence between contact force and non contact force

The force which acts on an object without coming physically in contact with it is called non-contract force. The most familiar example of a non-contract force is weight. In contrast, a contact force is a force applied to a body by another body that is in contact with it.

The speed of sound can change when sound travels from one medium to another. However, the frequency usually remains the same because it is like a driven oscillation and has the frequency of the original source. If vw changes and f remains the same, then the wavelength λ must change.