# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Dr Amrutha on 22/04/2016 09:42:42

Title: What is the weight felt by a person in freely falling lift ?
Post by: Dr Amrutha on 22/04/2016 09:42:42
It's zero [;)]  But I don't know how scientifically.Can someone explain this to me?
Title: Re: What is the weight felt by a person in freely falling lift ?
Post by: evan_au on 22/04/2016 11:29:31
Imagine that you have a mass of 60kg, and are standing on a set of glass bathroom scales, in a stationary lift/elevator. Your mass of 60kg, acting under a gravity of 9.8 m/s2 pushes down on the scales with a Force of 60x9.8=588 Newtons, and the scales display that force as a weight in pounds, stones/pounds or kilograms force (depending on the country).

Now imagine that the lift enters free fall, Your mass of 60kg, acting under a gravity of 9.8 m/s2, accelerates downwards at 9.8m/s2 (as do the lift and the scales). Let's say that random air currents place the scales 1cm above the lift floor, and your feet 1cm above the scales as you fall. There is no pressure of your feet on the scales, and no pressure of the floor on the scales, so the scales will read zero*.

Hence, your weight, in a freely falling lift is zero.
But watch out - your weight reappears all at once to make up for it's temporary absence, when you reach the bottom of the lift well!

*Actually, the scales will read slightly less than zero, because the scales are calibrated to ignore the mass of the glass plate on the scales - only the glass plate now weighs nothing.
Title: Re: What is the weight felt by a person in freely falling lift ?
Post by: Dr Amrutha on 22/04/2016 12:19:07
Let's say that random air currents place the scales 1cm above the lift floor, and your feet 1cm above the scales as you fall. There is no pressure of your feet on the scales, and no pressure of the floor on the scales, so the scales will read zero*.
I do not understand how you relate pressure to zero weight in the above statements.Can you please elaborate more on that ?
Title: Re: What is the weight felt by a person in freely falling lift ?
Post by: Colin2B on 22/04/2016 13:52:43
I do not understand how you relate pressure to zero weight in the above statements.Can you please elaborate more on that ?
Pressure is force/unit area so you can read force where Evan has written pressure without changing the sense of what he was saying.
Title: Re: What is the weight felt by a person in freely falling lift ?
Post by: PmbPhy on 22/04/2016 15:52:16
It's zero [;)]  But I don't know how scientifically.Can someone explain this to me?
The weight of an object can be and is defined in two ways;

1) the force due to gravity on a body
2) the force required to hold a body at rest, i.e. to prevent it from falling, in a gravitational field.

Let's use the second definition since most of us think that way. When you're in free fall there are no forces other than gravity acting on your body. The force of gravity is not something you can feel when you're in free fall because it causes every single particle in your body to fall at exactly the same rate. In your frame of reference, i.e. the frame in free fall, there is, for all practical purposes, no gravitational field. In fact in general relativity there really isn't a gravitational field in a free fall frame. It's said to have been transformed away.