2. Place the index card over the opening. It must be totally
covered.
3. Holding the card in place with the palm of your hand or fingers,
invert the glass. Slowly, remove your hand from the card. ( If you
are using a plastic glass. be careful not to squeeze the glass)
4. Observe.
WHY?
The water stays in the glass because when you take your hand away
from the card, it sags a little. The pressure inside the glass is
decreased just enough so that the air pressure underneath the card
is pushing up with enough force to hold the water in the glass even
though it is upside down. Also,the water in the glass, because of
surface tension, forms a slight suction between the glass and the
index card.
ASSESSMENT: Either of the extensions can be used for
assessment. Can the student communicate the findings?
4 . . . . The student is able to communicate that air exerts pressure
in all directions.
3 . . . . The student is able to communicate that something is holding
the water up.
2 . . . . The student is able to communicate that the card is stuck
to the glass.
1 . . . . The student is able to communicate that it's magic.
EXTENSIONS:
1. Put a ruler on the edge of a table, so that about 1/3 extends off
the table. Cover the 2/3's of the ruler on the table surface with a
large piece of paper. Hit the extended part of the ruler with your
hand. What happens to the paper? Why? The paper remains flat on
the table and does not fly up in the air as anticipated. The pressure
of the air is so strong it can't be lifted up. ( air pressure is
about 15 Lbs/sq.inch)
2. Put a straw into a glass of water. Remove it. Is the any water
inside? Now put the straw back into the water. This time, cap off
the top with your finger before removing the straw from the water.
What happens? (water remains inside the straw) Release your finger.
Now what happens? ( the water runs out because air pressure pushes
down as gravity pulls the water down.)
