When you apply a force on a material, such as
pulling, pushing, or twisting, that material can
either withstand the force and remain unchanged,
or it can change in some way, such as stretching,
crumpling, or bending. After the force on a
material is removed, some materials go back to
their original shape, like a stretched rubber band
returning to its original size. These materials
are known as elastic materials. Some
materials, however, don't return to their original
shape (or even fall apart), and we usually say
that these materials have broken.
Whether a material breaks usually depends on
how you apply force to it. For example, if you
take opposite ends of a large piece of paper and
pull your hands apart, the paper is surprisingly
strong--you can pull pretty hard without it
breaking. However, it's very easy to break the
paper in a different way, by ripping it!
In the case of a water balloon, even though the
balloon is full of water and stretched out, the
balloon is able to withstand that stretching
without breaking, and if you take the water out,
the balloon returns to its original size. If you
poke that full water balloon with a needle, now
instead of stretching the balloon material evenly,
you have created a place on the balloon where a
ripping-type force can occur, and the
balloon rips apart--it pops!
Balloons are able to stretch without breaking
because of the material they are made out of.
Balloons are usually made out of materials called
elastomers, which is a word that combines
two words: elastic and polymer. Polymers are
long, chain-like molecules that tangle together
and form materials with interesting
properties. For example, rubber is a
type of naturally occurring polymer, and
nylon is a type of man-made polymer.
One property that polymer materials can have is
elasticity. The rubber bands mentioned
earlier are polymer materials with this property,
and so are balloons, which are usually made of
rubber or latex polymers.
This elasticity is why balloons can
withstand being stretched without breaking …
at least until you poke them with a needle!
The strength of the material from which the
balloon is made determines how difficult it is to
pop. Filling a balloon with anything, whether
water, air, or some other substance, will stretch
the balloon material and subject it to tensile
(pulling) stresses. Once the stress exceeds
the strength of the material, the balloon will
In a filled water balloon that is stationary (not
moving), the balloon is kept from popping by
pressure from the atmosphere. Contrary to
what we might think as "intuitive", atmospheric
pressure does not just act downward on objects; it
acts in all directions - down, up, from the
sides and everywhere in between. This force from
all sides exerted by air molecules on the balloon
prevents the water in the balloon from breaking
the balloon itself.