# Homework Help: Simple question about zero gravity and weightlessness

1. Nov 23, 2013

### QuickDraw

Ok, I have to do my case study on zero gravity but I am just not understanding it. It says weightlessness is similar to when you are in a free fall, for example in an elevator because you are accelerating at gravity's force? I don't get what EXACTLY makes us weightless just because we are free falling.

2. Nov 23, 2013

### Kishlay

weightlessness happens when there is no force on you.....!!! at the time of free fall the force experienced by you is equal to 0

3. Nov 23, 2013

### Dick

If you are suspended three feet above the floor in a stationary elevator, you will fall the floor of the elevator. That's not weightless. If you are suspended three feet above the floor of the elevator and the cable holding the elevator is cut, then both you and the elevator will fall towards the center of the earth at an acceleration 9.8m/s^2. So you won't hit the floor of the elevator since you are both falling at the same rate. That's what's called weightless and free fall. You and the elevator are falling at the same rate. So you float in the elevator. It's not because there is no force acting on you. The same thing happens to astronauts in orbit. There is a substantial force on both the astronaut and the vehicle, but it affects both accelerations in the same way.

4. Nov 24, 2013

### QuickDraw

Is this a good way of explaining this then?

Zero gravity can now be manipulated on Earth similar to the way someone in freefall. Think of yourself sitting on a chair: the chair exerts an equal amount of force you push it with. You can feel the force for example from how your abdominal muscles hold your abdominal organs in place strong, or any other parts of your body, this is how it feels when you can experience weight. Now imagine if the chair exerted zero force, you would feel weightless. Another way of showing is a man inside of an elevator, and the cable of the lift breaks. Both the elevator and the man will be in free fall, which makes them feel weightless as they cannot feel the force with their abdominal muscles.

5. Nov 24, 2013

### vela

Staff Emeritus
Some textbooks call this apparent weightlessness to avoid the misconception that there is no force on you. You're not actually weightless in free fall because gravity is still acting on you; however, you appear to be weightless because everything around you, i.e. the elevator, falls at the same rate as you do.

6. Mar 22, 2014

### PhysicsGeek

It actually depends upon the frame of reference you are in. If there is no force acting on you, you will be weightless and will float. This is also what happens to astronauts in space. They are not free from earths gravity as such, only the effective force acting on them is zero. This video should help clarify things further: https://www.youtube.com/watch?v=HljNJE5CN6g&list=UUUCdU4xZCCNPtIAehT8z9vA

7. Mar 22, 2014

### Filip Larsen

Welcome to PF, PhysicsGeek.

I assume you have chosen to reply to this somewhat old thread because you disagree with the earlier explanations. So lets see ...

Weight of a given object is usually defined as the (negative of the) vector sum of all surface forces on that object, that is, disregarding volume forces like gravity. In practice, weight is the reading a weighting scale has when a mass is "placed" on it, and this read should be the same for all observers in all reference frames. In other words, weight is considered frame invariant.

For example, an astronaut can feel weight in outer space far from any gravitating sources if he is standing inside a steady accelerating rocket. Both he and any observer on earth will agree that he has weight in this situation. And in a similar vain, if a lift is free falling down a shaft with a person inside, all observers will agree that this person experiences no weight while in free fall.

A good example of true weightlessness in practice at (or near) the surface of Earth could be the "vomit comet" [1] and in orbit it could be a zero drag satellite [2], [3].

[1] http://en.wikipedia.org/wiki/Reduced_gravity_aircraft
[2] http://en.wikipedia.org/wiki/Zero-drag_satellite
[3] https://einstein.stanford.edu/Media/DF-satellite-flash.html

Last edited: Mar 22, 2014