Teacher Demonstrations
Straw through a potato
This quick and simple demonstration will amaze your students.
They can repeat it at home and amaze their family!
Materials: Plastic straws (sturdy ones are best) / Potatoes / Gardening glove (for safety)
Instructions:
y Propose that you are going to pierce a potato with a flimsy plastic
straw to the class … you could ask if anybody would like to bet $5
that you can it.
y Some students may suspect you are playing a trick so allow them
to inspect the straw and potato.
y Hold the straw firmly but without crushing it. Wear a glove on the
hand holding the potato. Hold the potato in your fingertips (as
illustrated) to allow the straw to visibly penetrate right through.
Note: There is no need to put your thumb over the end of the straw.
y Take aim and stab the potato as hard as you can! If you are having
trouble getting the straw all the way through, you’re probably
hesitating. Just pretend it is as rigid as a pencil and stab the potato
harder.
y Ask if anyone can explain how this simple trick works.
To create an environment where students feel safe to ‘have a go’, tell them your
first guess was wrong when you first saw this trick (even if it wasn’t!).
Some hints to encourage responses: “Well, what shape is a straw?” (cylinder) / “In
what direction are cylinders strong?” (end to end) / “Is the plastic thick or thin,
blunt or sharp?” (thin and sharp)
Explanation:
The straw penetrates the potato easily because it is a strong cylinder made from thin plastic which also
makes it sharp . Cylinders are strong if compressed from end to end, but not from the sides. Raw potatoes
are hard, but easy to cut with a sharp knife (or straw!).
A common misconception:
A common published misconception is that air pressure in the straw keeps it rigid allowing it to pierce
the potato. Be careful to give praise for this explanation before explaining that it is actually a common
misconception. To prove it has nothing to do with air pressure, repeat the demonstration with your
thumb clearly not covering the end of the straw. Now no air pressure can build up yet the trick still
works.
Safety note:
Even if you are confident of your aim, you should demonstrate the safety precaution of wearing a
gardening glove to protect your hand in case you miss the potato. Although an accident may be
unlikely, insist that students wear a glove if they decide to repeat this demonstration at home.
© Ruben Meerman 2004
Teacher Demonstrations
Unpredictable shoelace
Challenge your students to predict what will happen when you let go
of the keyring … not quite what you might expect!
Materials: Bunch of keys / Key-ring / Shoelace / Pencil or pen
Instructions:
y Tie your keys to one end of the shoelace and a single key-ring or
ring. A single granny knot is sufficient to tie them. You can do this
in front of the class … anticipation can be the best part of a
demonstration.
y Hold the single key-ring and let the bunch of keys hang over the
pencil as illustrated.
y Ask if anyone would like to predict what will happen when you let
go of the key-ring. If no-one responds, ask anyone who thinks the
keys will hit the floor to raise their hands.
y Let go of the key-ring. The ring should spin around the pen several
times causing the keys to remain hanging from the pencil.
Explanation:
This is could be thought of as a pendulum trick. A pendulum usually swings to the height from which it is
released. But as the keys drop toward the floor, the length of the shoelace hanging over the pencil on
the key-ring side gets shorter. By the time the key-ring has swung under the pencil, it is being lifted up by
the heavier weight of the keys falling on the other end. The key-ring ends up with much more kinetic
energy than is required to reach the height from which it was released so it overshoots and wraps
around the pencil. Friction takes over once the shoelace has wrapped around the string a few times and
stops the keys falling any further.
See also: Science Conundrums “Pendulum Problem”
© Ruben Meerman 2004
Teacher Demonstrations
A bounce plus a bounce?
Ask your students to do some simple bounce arithmetic
and they’ll spring to attention
Materials: Large ball (basket ball works best) / Small ball (a mini-soccer ball works best)
Instructions:
y Drop the larger ball from shoulder height and ask the class to rate
the height of the bounce from zero to ten (zero is no bounce at all
and ten is a bounce up to the shoulder)
y Repeat the bounce rating for the smaller ball.
y Now hold the two balls so that the smaller one is resting and
centred on top of the larger one. Ask the class how high they think
the smaller ball will bounce when you let them go simultaneously.
You could suggest that it might be the sum of the ratings they
gave the balls individually.
y Drop the two balls drop simultaneously. The smaller ball will shoot
much higher than the sum of the two ratings. In fact, it may easily
hit the ceiling!
y Repeat the demonstration a few times and ask the students to
observe what is happening to the larger ball. They should notice
that it hardly bounces at all now. Ask if anybody would like to
attempt an explanation being careful to praise any attempt as this
is a demonstration of a reasonably complex collision.
Explanation:
The physics of this demonstration is actually quite complicated – a mathematical description would be
beyond the scope of most first year university physics courses. But a conceptual explanation will be easily
understood your students.
You would have noticed that the larger ball hardly bounces at all in this demonstration. On its own, it
bounces a certain height and all this energy is now being transferred to the smaller ball during the
collision. Both the kinetic energy and momentum of a moving object depend on its mass (weight). A
heavier object has more kinetic energy and more momentum than a lighter object moving at the same
speed. So the energy received by the smaller ball makes it go faster because it weighs less.
Safety note:
Switch off ceiling fans before performing this demonstration. Try it on your own a few times first. The
smaller ball has a tendency to fly in any direction if not properly centred on the larger ball when it
collides with the ground. Put away any breakable items from the area where you are performing the
demonstration and have students sitting about 2 metres away.
© Ruben Meerman 2004
Teacher Demonstrations
Egg in the bottle … with a balloon
Parents will love seeing this demonstration revived in your classroom!
Materials: Large glass bottle or jar with wide mouth / Balloon / Water / Paper / Matches
Professor Julius Sumner-Miller made it famous in Australia with a milk
bottle and a hard-boiled egg. Milk bottles have since become obsolete
but fortunately, a water-filled balloon works just as well and you can
use bottles with a top of almost any diameter.
Instructions:
y Fill a standard balloon with water so it is a few centimetres wider
than the mouth of your glass bottle and tie a knot.
NOTE: if the balloon is too big, it will go part of the way into the bottle
and stop.
y Tear a small piece of paper about 10 cm long and 2 cm wide.
y Light the piece of paper with a match and let it burn for a second
or so, then drop it into the bottle. Let the paper burn for about a
second in the bottle and then immediately place the balloon on
top of the bottle. The outside of the balloon should be a little bit
wet with water to help it slide into the bottle.
SAFETY NOTE: You could hold the paper with tongs if desired.
y The balloon should get ‘sucked’ into the bottle fairly quickly. If it
gets stuck, the balloon may have had a little too much water in it. It
looks funny if this happens and the students will probably get a
giggle out of it. In this case, it’s best to laugh along with them, ask
if anyone can suggest why this happened, then pull the balloon
back out and try again with a new balloon and less water.
continued …
© Ruben Meerman 2004
Teacher Demonstrations
Egg in the bottle … with a balloon
continued
y If you want to repeat the demonstration, you should be able to
remove the balloon from the bottle without too much difficulty.
Wet the balloon a little so it glides out. If it does get really stuck,
just hold the bottle upside down, pierce the balloon and catch the
water in a bucket. This avoids getting too much water in the bottle
which might extinguish the flame prematurely when you try again.
Explanation:
The burning paper generates heat. When air heats up, it expands. While the balloon is resting on top of
the bottle, the expanding air pushes it up and escapes. Once the flame dies out, the air inside the bottle
starts to cool and contract but the balloon is blocking the entry of air. As the pressure inside drops, the
higher pressure outside forces the balloon into the bottle. The balloon acts like a one way valve allowing
air out, but not into the bottle.
Safety note:
Before you perform the demonstration, emphasise any safety precautions you are taking (eg, have some
water in a bucket and a damp towel nearby). Insist that students only perform this demonstration under
adult supervision and exactly as demonstrated paying particular attention to the size of the piece of
paper used.
© Ruben Meerman 2004