|Hi there! I am interested in teaching second
graders about why a fully inflated ball bounces
better than a less inflated ball. I saw you
already have this question on your website and
I'm wondering if you can explain this in second
grade terms. Thank you so much!
|Question Date: 2019-07-05|
A ball filled with air is basically a
spring. Pushing the air molecules closer
together (i.e., fully inflating the ball) makes
the "spring" stiffer. When the ball is about to
hit the ground, it has some energy (capacity to do
something). Because the ball is moving, this
energy is called kinetic energy. As the
ball hits the ground, the kinetic energy is stored
as potential energy in the "spring" as the ball
deforms and the air inside is compressed. But
some of the kinetic energy is lost to friction
(becoming heat) as the skin of the ball is
deformed. Thus, less energy will be available
to push the ball back up and it will not bounce as
high. In a partially-inflated ball, the air
molecules are not very close together, so pushing
them nearer each other (when the ball hits the
ground) is relatively easy. In a fully-inflated
ball the air molecules are already packed close
and further reducing that separation is difficult.
Thus, the less inflated ball will deform more than
the fully inflated ball, in the process losing
more useful energy to friction, and then having
less energy to bring the ball back up.
here for more information.
Very good question! The answer lies in Isaac
Newton’s third law. The law states that for
every action, there is an equal and opposite
reaction. That means when you sit in your
seat, the seat pushes back to hold you up so you
don’t fall right through the chair. Now let’s
consider a ball, like a basketball. The inside of
the ball only has so much space, so the more air
you pump into it, the more squeezed together the
air gets. This is called air pressure. Now
the air inside is pushing so hard on the skin of
the ball that when it hits the ground, it hits the
ground really hard! And remember Newton’s third
law? If the ball hits the ground really hard,
the ground is going to hit the ball back just as
hard. That’s why the less inflated ball won’t
bounce as high. The air particles inside aren’t as
squished and are just fine with hanging out inside
the ball with minimal effort to push on the
It's because the air inside of the inflated
ball is under pressure, while the air
inside of the deflated ball is under less
pressure. Pressure means force, and Newton's third
law says that when you push on something, that
something pushes back. You can give an example of
pushing on a wall: if you push on a wall, the wall
pushes back to keep you from going through it. If
you push on the wall hard enough, you will push
yourself away from the wall. The inflated ball
when it hits the ground it pushes so hard that the
ground pushes the ball back up into the air - in
other words, it bounces.
What a fun challenge! Explaining this physics
to second graders!
The less inflated ball is sort of squishy, so
when it hits the floor, some of the energy goes
into squishing the ball, and there's not so much
energy left for making the ball bounce back up.
The fully inflated ball is stiffer or harder, so
when it hits the floor, pretty much all of the
energy goes into making the ball bounce back up.
Newton's Cradle could show this - stick things to
a ball in the 'cradle' and see how it affects the
bouncing. Tape vs clay etc. They things would
damp the balls' bouncing by various amounts. But
2nd graders could quickly wreak havoc on an
unattended 'cradle.' A young neighbor kid messed
with my last one till it was tangled. Luckily it
was a tiny cheap 'cradle' so I didn't care.
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