Weight is the pull of gravity. Typical weight units are pounds and
Newtons. (1 pound @ 4.45 newtons). On the moon, gravity pulls
with 1/6 the
force compared to the Earth. Therefore, a student on the moon weighs
1/6 of what
she weighs on the Earth.
On the Earth, neglecting air resistance, all objects will speed up
at a rate of 9.80 m/s every second they fall. That is a speed increase
of about 22 mph for every second an object falls.
Time in the Air
There are two ways to experience weightlessness. (1) move
far enough away from the planets and sun to where their pull is
nearly zero. [Gravity acts over infinite distance. One can
never completely escape it.] (2) Fall down at a rate equal to
the pull of gravity. In other words, accelerate to the Earth speeding
up 22 mph every second in the air. In order for a person to feel
weight, a person must sense the reaction force of the ground pushing
in the opposite direction of gravity.
In the absence of the reaction force a person will sink through
Many amusement park rides generate the weightless sensation by
accelerating down at 22 mph every second.
Neglecting air resistance, if a rock is dropped, it will
accelerate down at 9.8 m/s2. This means it
will speed up by 9.8 m/s for every second it falls. If a rock you drop
accelerates down at 9.8 m/s2, scientists say
the rock is in a "1 g" environment, [1 g = 9.8 m/s2 = 22 mph/s].
Any time an object experiences the pull equal to the force of
gravity, it is said to be in a "one g" environment. We live in a 1 g
environment. If a rock whose weight on the Earth is 100 lbs was moved
to a 2 g environment then it would weigh 200 lbs. In a 9 g
environment it would weigh 900 lbs. In a "NEGATIVE 2 g" environment
it would take 200 lbs to hold the rock down on the ground. In a "-5
g" environment it would take 500 lbs to hold the rock down to the
ground. If the rock were put into a "zero g" environment then it
would be weightless. However, no matter what happens to its weight
the rock's mass would never change. Mass measurement is
unaffected by the pull of gravity.
What does it feel like to walk in a 2 g environment? Have students
find someone who's mass is about equal to theirs. Have them give
piggyback rides. As they walk around this is what it feels like to be
in a 2 g environment. Go outside on the soft ground and have the
students step up on something. This is when they will really know
what a 2 g environment feels like.
Often engineers will use g's as a "force factor" unit. The force
factor gives a person a way of comparing what forces feel like.
All acceleration can be converted to g's by dividing the answer, in
m/s2, by 9.8
A roller coaster is propelled horizontally by a
collection of linear accelerator motors. The mass of the coaster
train is 8152 kg. The train starts from rest and reaches a velocity
of 26.1 m/s,
55 mph, in 3.00 seconds. The train experiences a constant
acceleration. What is the coaster train's acceleration in g's?
m = 8152 kg
vo = 0 (starts from rest)
vf = 26.1 m/s
t = 3.00 s
a = ?
vf = vo + at
26.1 = 0 + a(3.00)
a = 8.70 m/s
in g's... 8.70 m/s2 / 9.80
m/s2 = 0.89
This means the rider is being pushed back into his seat by 89% of
1 plastic cup
2 skinny 4" diameter rubber bands
2 50 g masses
Cut the rubber bands. Tie the ends of each together to make a
stretchy string. Tie the weights to the opposite ends of the rubber
band. Attach the middle of the rubber band to the inside bottom of
the cup. The two masses should be able to hang over the lip of the
The masses are in equilibrium with the upward force of the rubber
band. The force pulling up of the rubber bands is equal to the force
of gravity, [weight of the masses.] Ask the class, "What would happen
if the rubber bands pulled with a force greater than the pull of
gravity on the masses?" The masses would shoot upward and be pulled
into the cup. To show this, pull down on one of the masses and let
Now ask, "What would happen if the masses could be magically shielded
from the pull of gravity?" With no force stretching the rubber bands,
they would sling shot into the cup.
Explain, "We cannot yet shield gravity. But we can momentarily
minimize its effects by accelerating the masses, rubber band and cup
down at 9.80 m/s2, the
acceleration due to gravity." Without saying anymore, stand on a
chair. Raise the apparatus with the weights hanging out. Tell the
students, "When this cup is dropped everything will speed up equal to
the acceleration of gravity. What will you see when this cup is
Drop the cup after polling the students. The masses will be pulled
into the cup. When everything falls, gravity will not be pulling
against the masses when compared to the rubber band's pulling
force. The masses are said to be weightless. It is the weight of the
mass that stretched the rubber band. If the mass is weightless, the
rubber band will pull it in.
When everything is falling together, the pull of gravity is no
longer experienced by the rubber bands. Therefore, they pull the
masses into the cup.
With the balloon inflated, hold the frame over a pillow. Hold the
frame straight out at chest height. Ask students to predict what will
happen when you release the frame. Guide them to specifics such as
where in the fall will the balloon pop. Release the frame. The
balloon will pop almost instantaneously. The balloon pops because the
weighted pin becomes weightless. The rubber bands are essentially
pulling against nothing. This means the rubber bands pull the pin up
into the balloon.
WEIGHTLESS DEMO #3
Heat up the nail with the candle flame. Be careful not to burn
yourself. Poke the hot nail into opposite sides of the cup at the
bottom. This will make a clean hole. Hold your fingers over the two
holes and fill the cup half full of water.
Stand on a chair and briefly release your fingers from the holes.
The water should stream out the cup's holes onto the floor. Ask the
students, "What will happen when I drop the cup into the trash can?"
Listen to all their answers. Drop the cup to see who's prediction was
correct. The water will not flow out of the cup. Water flows out of
the cup when the acceleration of the cup is different from the water.
When the cup is held the water is allowed to accelerate down at 9.8
m/s2. When the cup
falls too, the cup is also accelerating down at 9.8 m/s2. Since there is no difference in their
accelerations the water stays in the cup.
If you use or find this page useful or have any
comments, please contact the author so maybe he'll do more.
Author: Tony Wayne
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