Question

when two objects come in contact with each other why they make sound. give scientific reason.

Answer 1

The contact force between two objects is nonlinear and piecewise (typically Hertzian). This type of force results in a very short peak of force (a pulse). Such a sharp peak contains energy at a broad range of frequencies, which excite the resonances of the two bodies that collide. The bodies then keep resonating and their vibration radiates sound in contact with the air, this is most of the sound that we hear. 

Depending on the bodies, the sound will be different (Think of a Xylophone being struck by a mallet, the different bars produce different sounds), because they have different resonance frequencies.
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