So last week, we talked about how the effects of Gravity, Momentum,
Friction, and Density affects a basketball/soccer ball. We learned
that as a ball bounces, the effects of its kinetic energy vs. gravity
is what gives that signature tapered arc in the bounces, and that a
ball has height degradation, which means that each bounce causes
the ball to lose energy, so that each consecutive bounce will be
smaller that the previous one, unless it somehow gains energy, like
from falling off a ledge.
Now that we know all that, how do we then apply it to balls of
different weights? What is it that really makes the difference
between a ping pong ball and a cannonball? Is it size? A bowling
ball weighs more than a tennis ball, but on the flip side, a small
lead ball weighs a lot more than a large beach ball. So that's not
it. Maybe it's gravity pulling on one ball more than the other? You
can actually experiment with this yourself by taking two objects of
different weights (preferably not something sharp or fragile) and
dropping them at the same time. You'll find that no matter the
difference in weight, both objects will hit the ground at the same
time. Gravity has the same pull on any object, no matter it's size
or weight. But then why do balloons, feathers, and sheets of paper
float down to the ground instead of dropping? The reason for this is
air friction, air density, and the surface area of an object which
we'll discuss a bit further down.
The answer to our question is Mass and Density. An object's mass is what
determines how heavy it is, and consequently, how its arcs and height
degradation is affected. Density is what causes different objects of
the same size to weigh more or less than each other. A tennis ball has
a small amount of mass in a relatively small amount of space, making it
light, whereas a tennis ball sized lead ball has a large amount of mass
in a small amount of space, making it very dense.
Mass & Density on a Bouncing Ball
So how do these affect the bouncing ball? As I mentioned last week,
when a dropped ball bounces, the downwards energy it got from falling
(gravity) gets converted into upwards energy which then has to fight
against gravity to be able to get the ball into the air again. With
the size and drop distance of both a light and heavy ball kept constant,
the heavier, denser ball will fall with more energy (not to be confused
with more speed). The reason for this is because gravity accelerates
all things equally, regardless of mass, and momentum = mass * velocity.
So the denser ball has more momentum and more kinetic energy as it
strikes the ground. In fact, the reason why denser objects overcome air
drag better is that they have more momentum.
With that information, you would think that a heavy ball would bounce
higher than a light ball because of the increased momentum and energy.
Depending on the surface and material of the ball, it definitely can
(Bowling balls on concrete can bounce pretty well). But the reason why
heavy balls often bounce less is that when they impact, they are more
likely to lose energy into the ground plane because of their increased
mass and momentum.
But now what about ball roll? How do heavy balls roll compared to light
balls? In general, a light ball doesn't take much energy to start
rolling, but a heavy ball will roll faster and longer than a light
ball, the more momentum it has. This is the same reason a large
semi truck takes forever to stop compared to a small car. Be sure to
ask your mentor what kind of rolling they like to see on the heavy
balls. Some mentors, like mine, wanted to see a heavy ball drop almost
straight down with no roll to emphasize the weight, but others like
Beau's wanted to see a heavy ball with a longer roll to show momentum.
Light Balls VS Balloons
I bet you thought I forgot about this topic! Nope. I've left it for
last because it doesn't really have anything to do with light vs heavy
balls, but is still kind of an important topic to talk about. As I
mentioned before, the reason why light balls drop and balloons, feathers,
and sheets of paper float down to the ground isn't because of gravity
pulling things down with different amounts of force, but because of air
friction, air density, and the surface area of an object.
Balloons float down so lightly because they have so much more air inside
them than they have the rubber that makes them, and the rubber is
distributed over such a large area that its density is extremely low,
causing it to float down. You can see that a balloon has a density similar
to air by the fact that you can't throw it or hit it very far. Now Paper
and feathers "float" because their wider surface areas and light weight
cause them to glide over the air like a hang glider would, not because of
air density. If you were to drop a piece of paper on it's edge, you'll see
that it slices through the air and drops pretty quickly, or if you crumple
it, it will drop like any other object.
Thank you for taking the time to read my post. I hope it's been helpful,
and as always, if you have any comments, questions or suggestions,
you're more than welcome to leave a comment or send either Beau or
me a message on AM.
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