› › › PHYSICS EXPLANATIONS
Gumball Physics
Here is an overview of the tracks and stunts included
in this kit. All of these parts attach to the pegs on the
sides of the tower, and each one can be used to
demonstrate physics principles.
1
The
180-degree smooth track is the simplest
track segment. A gumball rolls from the top to the
bottom, 180 degrees around the tower. This simple
motion demonstrates many things. The gumball has
mass.
Mass is the quantity of matter in an object or a
body, which is a physics term for a physical thing.
Matter is any physical substance that occupies
space. Mass can also be thought of as the ability of a
body to be heavy. Mass should not be confused with
weight though.
Weight is a measure of the force that
gravity exerts on mass.
A
force is the cause of a change in a body’s state
of movement. A force can be thought of as a push or a
pull on an object. A push of the gumball causing it to
roll across the table is a force.
Gravity is also a force. It is Earth’s force of
attraction on mass. Gravity is a fascinating thing: All
mass attracts each other. The larger the mass, the
greater its force of attraction. Earth has a huge mass:
5.9 trillion trillion kilograms, or 13 billion trillion
tons! The gumball has a very small mass: about two
grams. Therefore, Earth pulls the gumball toward it.
This is the force of gravity!
A gumball rolls down the track because gravity
is pulling it. On Earth, the gravity at the surface is a
downward force that causes an acceleration equal to
about 9.8 meters per second per second (m/s/s or m/
s
2
).
To understand acceleration, we have to
understand speed.
Speed is the distance traveled by a
body in a certain amount of time.
Velocity is a
physics term for speed that also takes into account
the direction of motion.
If a gumball travels five inches in one second, its
speed is five inches per second, or 5 in/s — which is
equal to about a quarter of a mile per hour.
Acceleration is the measure of the change in
speed (or more accurately, velocity) over a certain
amount of time. So, what is gravity again? Earth’s
gravity causes a body to accelerate to a speed of 9.8
meters per second for every second that gravity acts
on the body — regardless of the mass of the body. If
you drop a gumball and it falls for three seconds, by
the end of its fall, it is moving at a speed of 29.4
meters per second (9.8 m/s
2
x 3 s = 29.4 m/s).
As the gumball rolls down the track, gravity
accelerates it and it moves faster, unless it
encounters obstacles.
Amazing! This one simple segment of track has
allowed us to define all these physics terms.
2
The
momentum trap track demonstrates a few
more important physics principles. The momentum
trap looks like the 180-degree straight track, but with
one important difference. It has a little “speed bump”
in the middle of it. If you place a gumball on this
speed bump (or even in the little trough in front of the
bump) and release another gumball to roll down from
the top of the track, the rolling gumball will collide
with the stationary gumball. The stationary gumball
will then be knocked into motion and continue down
the track, while the previously moving gumball will
now be stuck in the trough. To understand what
happened here in terms of physics, you have to
understand momentum and inertia.
Inertia is the tendency of a body to remain at rest
or in motion. It can also be thought of as the amount
of resistance to a change in velocity. The more mass a
body has, the more inertia it has. Since both gumballs
have the same mass, they have the same inertia.
Momentum is the combined effect of the mass
and velocity of a body. All moving bodies have
momentum. A fundamental rule of physics says that
momentum is always conserved when two bodies
collide. When the gumballs collide, the momentum
from the moving gumball is transferred to the
stationary gumball, causing the latter to move. The
first gumball loses its momentum and slows down.
This describes a perfect
elastic collision. If the bodies
were to deform, or change shape, on impact, the
result would be different. This is an
inelastic collision.
The momentum trap can also be used to show
how a gumball must have a certain amount of
momentum if it is to make it over the hill. If it has too
little momentum, it will get stuck in the trap.
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