Apparent WEIGHTLESSNESS

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.

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.

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In the absence of the reaction force a person will sink through the ground.
Many amusement park rides generate the weightless sensation by accelerating down at 22 mph every second.

g's
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 ^m/s².

Example

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?

Solution

m = 8152 kg
v_o = 0 (starts from rest)
v_f = 26.1 ^m/s
t = 3.00 s
a = ?

v_f = v_o + 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 g's
This means the rider is being pushed back into his seat by 89% of his weight.

WEIGHTLESS DEMO #1

MATERIALS
1 plastic cup
2 skinny 4" diameter rubber bands

2 50 g masses

BACKGROUND

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 cup.

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 go.

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.
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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 dropped?"
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.

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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.

WEIGHTLESS DEMO #2

The DEMO
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



PREPARATION
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.

THE DEMO
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.