Define pressure. Give its SI unit.
The total force applied per unit area is called pressure.
Mathematically,
Pressure (P) = Force (F) / Area (A)
Its SI unit is pascal (Pa).
How does an air pump work?
Most hand pumps used to inflate bike tires are reciprocating pumps. They have a piston inside and two one-way valves: one going to the bike tire and one to the outside air. When the pump is pulled up, the pressure inside decreases. This pulls in air from the outside but closes the valve to the bike tire. When the piston is pushed down again, it compresses the air inside. This closes the inlet valve and opens the valve to the tire, pushing air into the bike.