Air is being forced by the bicycle pump into a tyre against a pressure of 4.5 bars. A slow downward movement of the piston can be approximated as

Concept:A pump moves slowly in downward direction follows Boyle's law.Boyle's law simply describes the relationship between the pressure and volume of an enclosed gas when Temperature remains constant. That relationship, USUALLY expressed as P1 V1 = P2 V2, means the product of pressure x volume remains UNCHANGED as either or both are changed.Since product of pressure and volume remains constant, for example, DOUBLING the pressure on an enclosed gas will reduce its volume to half its previous size.Boyle's Law is follows in a bicycle hand pump. By pushing down on the piston, the volume of the gas DECREASES, the molecules are now having more chances of collisions with the interior walls of the pump and this increases the pressure of the air inside so that it is forced into the tire.