杏吧原创

Deflation theory

Blow up two identical balloons to different sizes and pinch the ends. Connect
their mouths with a piece of plastic or rubber tube (you may need two pairs of
hands for this). Now let the air flow through the tube. What happens?

If intuition told you that the balloons would equalise in size, you鈥檇 be
wrong. The small balloon forces its air into the bigger one. The pressure inside
a balloon鈥攁s in a bubble鈥攊ncreases as its radius decreases. Try
listening to what happens when you stretch the mouth of a balloon over a party
squeaker. As the balloon deflates, the squeaker鈥檚 pitch rises as air is forced
out faster.

Ball bearings

Tie one end of a piece of string to a stone and attach a ping pong ball to
the other. Sink the stone slowly into a jar of water so that the ball floats
beneath the surface. Now put the lid on, and move the jar sharply to the right.
What happens?

The ball moves right鈥攖he same way as the jar. One way to understand
this behaviour is through Einstein鈥檚 principle of equivalence, which says that
acceleration in one direction is equivalent to a gravitational force acting in
the opposite direction. When you accelerate the jar to the right, it is as if an
apparent 鈥済ravitational鈥� force acts to the left. And just as a ping pong ball
floats upwards, against gravity鈥檚 direction of action, so in the experiment the
ball moves right.

Now try carefully spinning in a circle, holding the jar at arm鈥檚 length. The
ball moves in towards you, not away as you might have thought. The apparent
gravitational force in this case is towards the outside of the circle.

Swell egg

Place a raw egg in a jar of vinegar and leave it for a couple of days. What
happens?

The shell disappears and the egg swells up to perhaps twice its size. The
shell (calcium carbonate) reacts with the acetic acid:

2H⁺ + CaCO₃ 鈫� CO₂ + H²O + Ca²⁺

Next, water passes into the egg through its exposed membrane, driven by
osmotic pressure. The white of the egg has a very high concentration of protein,
and water passes from the vinegar into the egg in an attempt to equalise the
concentrations on either side of the membrane.

Now put the egg into a solution of 75 per cent syrup and 25 per cent water.
After two days, the egg should have shrunk to less than its normal size as
osmosis works in the opposite direction.

Which way round?

Put a yo-yo on a horizontal surface with the thread coming from the bottom of
the spindle. If you pull the string to the right, which way will the yo-yo
move?

The answer is left, right or not at all鈥攄epending on the angle q at
which you pull the string. When q is small, the thread tends to turn the yo-yo
anticlockwise. But a frictional force acts to the left of A, creating a torque
that opposes the rotation. Eventually, the yo-yo turns about A鈥攖hat is, it
rolls left. When q is large, the yo-yo still wants to rotate anticlockwise, but
there is also a strong lateral force to the right. Again friction acts to the
left of A, and this time it doesn鈥檛 just oppose the anticlockwise motion: in
combination with the lateral force it turns the yo-yo clockwise鈥攊t rolls
right. Between these two extremes, the yo-yo will simply spin without moving one
way or the other.

Flying high

Just by dropping a small superball from a height of 1 metre, can you make it
bounce high above your head?

You can if you drop it together with a larger superball, with the smaller one
on top. Assuming that the collision is perfectly elastic, the small ball will
bounce off at nearly three times the velocity with which the two balls hit the
ground. The final height of the small ball will be nearly nine times the
original height.

If you drop a stack of eight, progressively smaller superballs from 1 metre
then, in theory, the top one will fly off at a velocity close to 10 000 metres
per second, fast enough to escape the Earth鈥檚 gravitational attraction. In
practice, the air will slow down the ball鈥攊f the heat created by friction
doesn鈥檛 vaporise it.

Kitchen cookery

Dilatant materials are spooky. Treated gently, they flow like a liquid, but
put them under stress and they expand and act like a solid. Mix cornflour and
water until it becomes hard to move the spoon (start with 50 millilitres of
water and about 2陆 tablespoonfuls of cornflour). If you gently tip the bowl,
the mixture flows. But if you pull your fingers through the mixture it forms a
ball of putty-like material that you can work in your hands. If you now open
your hand, the ball reverts to a liquid and flows away.

Here鈥檚 another recipe for a dilatant. But be a little careful. One of the
ingredients, borax, is toxic and should be kept out of reach of children. And
watch the food colouring on wallpaper, clothes and carpets!

In a plastic cup, mix 20 millilitres of water, a couple of drops of food
colouring, and 25 millilitres of glue containing polyvinyl alcohol. PVA glue is
sold at toy shops and stationers. In another plastic cup, make up a 5 per cent
solution of borax, which is available at chemists. As a rough guide, try 5 grams
of borax in about 100 millilitres of water. Next, add 5 millilitres of this
solution to the other cup and stir with a lolly stick or plastic spoon. Leave
for three minutes, then pull out the solid material and throw away the rest.

You can roll the solid into a ball that will bounce on a hard surface. When
you pull it hard, it snaps. But leave it overnight and it will flow, taking up
the shape of its container. PVA is a long polymer that flows in liquid form.
When borax (Na₂B₄O₇.10H₂O) is added, the
borate ions form crosslinks between the polymer to create the dilatant gel. If
you want to keep the gel, store it in a plastic bag.

Motor magic

Once connected to the battery, the coat hanger, copper wire, salt water and
foil dish form a circuit. When the current flows through it, the wire in the
magnet鈥檚 field experiences a force. And because the field is symmetrical about
the axis of the cylinder, the wire is propelled round and round the magnet.

Cylindrical magnets are sold in toy shops and, believe it or not, by
suppliers of veterinary equipment. Fridge magnets generally do not work.