(figure available in print form)
The thrust (power) of an aircraft’s engines pushes it forward. At the same time, the movement of the wings through the air produces lift (a lifting force) that carries it up. A balloon also has lift but the wind pushes it forward through the air.
The pressure of the air around us is caused by the weight of all the air above our heads pressing down on everything on the ground. It is equal to a mass of about 1 kg placed on every sq cm of a surface. We do not normally feel this pressure because the air inside our bodies is at the same pressure.
We can increase the pressure of air inside a container, for example, by heating it or pumping air into it. If the container can expand, like a balloon, the increase in pressure will make it get bigger. If we lower the pressure, the container will get smaller if possible.