Physics Tricks
1.Cloud In A Bottle
Now-you-see-it, now-you-don't! This is a good trick for people to try themselves,
watching a cloud appear and disappear before their own eyes.
Ingredients
• plastic bottle with cap (fairly flexible e.g. from most soft drinks)
• water
• match
Instructions
1. Place a splash (~1 teaspoon) of water into the plastic bottle.
2. Light the match and make sure it is burning well, then drop it into the bottle.
3. Quickly screw the cap on, and squeeze the bottle with your hand five or six
times (for larger bottles you may have to do it slightly more). You should see a
cloud form in the bottle, then magically disappear when you squeeze it.
4. Pass the bottle around the audience to give everyone a chance to
experience it for themselves.
How does it work?
Clouds are formed when water droplets in the air cool and then collect on dust
particles. In this demonstration, the dust particles were provided by the smoke from
the match. The air inside the bottle was cooled by releasing the pressure after the
bottle was squeezed. The temperature is changed by squeezing the bottle: the
amount of air within the bottle is constant, but squeezing the plastic bottle changes
the volume of the gas. Expanding the bottle causes a lowering of the air
temperature - in this case, enough to cause the water gas to form a liquid - the
cloud.
Tips for Success
Try adding a small amount of food colouring to the water - it can help to increase the
visibility of the effect.
Serving Suggestions
This works well for small groups of people of all ages. It is particularly applicable in
outside environments where you can actually see clouds and potentially discuss the
science behind them.
2.Lifting Lemon
Levitate a slice of lemon using a few simple ingredients found in any pub.
Ingredients
• slice of lemon
• four matches
• pint glass
• ashtray
• water
Instructions
1. Pour water into the ashtray until the water is ~1cm deep.
2. Push three matchsticks into the slice of lemon, in the shape of a triangular
pyramid, with the match heads together at the top point of the pyramid.
3. Place the lemon and matchsticks in the centre of the ashtray, so that they
float on the water.
4. Light the fourth match and use it to light the other three together.
5. Invert the pint glass over the lemon and matches, letting it sit inside the
ashtray.
6. Watch as the lemon slice magically levitates within the pint glass!
How does it work?
The simplicity of this trick belies the complexity of the physical processes that
contribute to the effect¼
Firstly, there is a simple air pressure effect caused by the expansion and contraction
of the gas within the pint glass as it heats up and cools down. The heat from the
three matches causes the air inside the pint glass to get hot. When all the oxygen
within the glass is exhausted, the matches go out and the air inside the glass cools
down. The cooler air takes up much less space, so water gets sucked up into the
glass to take up that extra volume.
Secondly, the combustion reaction changes the species present, thereby changing
the volume of gas within the glass. When the matches burn they consume the
oxygen from the air within the pint glass. The products of this reaction are carbon
dioxide and water. The water will be a liquid, thus there will be less gas in the glass,
causing the water to be sucked up into the glass to fill the volume.
Tips for Success
The lemon slice needs to be thick enough to support the matches, yet still able to
float on the water.
Serving Suggestions
This trick was first developed for use in pubs, and draws on ingredients commonly
found in most pubs. However, there's no reason not to use it in other environments
also. It is most suitable for demonstrating to small groups of onlookers - it won't attract
a crowd by itself, but will maintain the interest of audiences that you are already
engaged with.
3.The Power Of Words
This demonstration challenges common preconceptions about forces, and
demonstrates the strength of atmospheric pressure.
Ingredients
• table with flat edge
• ruler
• newspaper
Instructions
1. Lay the ruler over the edge of the table so that ~1/3 of it's length is over the
edge.
2. Ask your audience what will happen if you hit the ruler from above.
3. Hit the ruler - as expected it flips off the table.
4. Ask your audience how you might possibly keep the ruler on the table while
you hit it, using only newspaper. Hopefully someone will guess that you need
to exert an opposing force on the far end of the ruler - you may need to
prompt them.
5. Tell your audience that you can only use a sheet of newspaper. Try first by
folding up a sheet of newspaper as small as possible and placing it at the
back end of the ruler so that it acts as a counterweight. Get an audience
member to hit the ruler again - still it flips off the table, this time along with the
folded up newspaper!
6. Ask your audience how else you might be able to use a sheet of newspaper
to hold the ruler down. If your audience guess the trick, ask them to explain
the physics behind the idea. Lay a single sheet of newspaper flat on the table
so that the ruler is roughly in the center. When you hit the ruler it will stay on
the table!
How does it work?
It all comes down to air pressure. Atmospheric pressure is exerting a downward force
on the single sheet of newspaper. The area of a single sheet of newspaper is fairly
large, therefore the downward force of the atmospheric pressure exerted on the
newspaper is strong enough to counter the upward force of hitting the ruler. It didn't
work with the folded-up newspaper because the surface area over which the
atmospheric pressure could act was far too small.
Tips for Success
For optimal effect, make sure as little air as possible is under the newspaper by
smoothing it out flat prior to hitting the ruler.
Serving Suggestions
This works well with audiences of all ages, but does require a sturdy table or bench, so
is best suited for performing when an audience has already been gathered.
