2. Atmospheric pressure is the pressure exerted by the weight of air in the
atmosphere of Earth (or that of another planet). In most circumstances
atmospheric pressure is closely approximated by the hydrostatic
pressure caused by the weight of air above the measurement point.
3. Ping-Pong Funnel. Put a ping-pong ball inside the
wide part of a funnel and blow hard into the narrow
end. Why doesn’t the ball pop out? As you blow into
the funnel, the air moves faster and lowers the air
pressure underneath the ball. Because the air pressure
is higher above the ball than below it, the ball is
pushed down into the funnel—no matter how hard
you blow or which direction you point the funnel.
4. Water Glass Trick. Fill a cup one-third with water. Cover
the entire mouth with an index card. Holding the card
in place, take the cup to the sink and turn it upside
down. Remove your hand from underneath. Voilà!
Because the water inside the cup is lighter than the air
outside, the card is held in place by about 15 pounds of
force from the air pushing up, while the force of the
water pushing down is only about one pound of force.