Homework Help: Weightless effect

1. Jun 10, 2015

Physiophysio

1. The problem statement, all variables and given/known data
(NB: posted earlier, but didn't agree with PF policy, so reposting as requested; apologies).
When a plane is in a steep dive, people say they feel ''weightless''. Do they in reality? Explain your answer. What do they actually mean by ''weightless''?

2. Relevant equations
W=mg

F = (G * m1 * m2)/(r^2)
I am not sure whether any other equations can be used?

3. The attempt at a solution
So, no, they are not weightless because there are other masses around exerting a force on them.

But I am not sure why we perceive it as ''weightless''. Could anyone help with this aspect of the question? Thanks in advance!

2. Jun 10, 2015

haruspex

If you are standing or sitting, in what way do you sense your weight?

Last edited: Jun 10, 2015
3. Jun 10, 2015

andrewkirk

They only feel weightless if the plane is accelerating towards the ground, as it will be towards the beginning of such a dive, but that will dissipate as the plane achieves terminal velocity. Think about the net forces experienced by the people in the plane during that acceleration phase.

4. Jun 11, 2015

CWatters

Technically that might be true but I suggest working out what those forces might be (eg what's the gravitational pull between two people?) and compare that of the earths gravity. Have you ever felt attracted to the person sitting next to you :-)

5. Jun 11, 2015

Physiophysio

Ah...you feel it because of the force produced from the ground you are standing on or the seat you are sitting on. But in a steep dive, the plane and you are both accelerating downwards, but in such a way that you are not accelerating down faster than the plane, (so equal to its acceleration or less) which means the seat of the plane is not in contact with you which is why nothing is producing a force on your bottom so you feel ''weightless''...is that right?

6. Jun 11, 2015

Physiophysio

That is true:
I mean, if
[G(m1)(m2)]/(r^2),
is equal to force exerted by one mass on another,
then my mass and that of the person next to me is rather tiny, and as G is a very, very small number, the final outcome of that equation is very tiny...so much so that friction and the earth's gravitational pull overcome it.

7. Jun 11, 2015

Physiophysio

So...in a steep dive, the plane's weight is larger than lift, so there is a resultant force downwards so it accelerates downwards. The same cannot be said of the people inside the plane, which is why people feel weightless because the seat they are sitting on is not in contact with them as it is accelerating downwards faster than they are...so there is no reaction force on a person to their weight, which is why we feel weightless...as we perceive our weight through the reaction force of the ground or seat on us. Right?

8. Jun 11, 2015

haruspex

Exactly.

9. Jun 11, 2015

jbriggs444

You've got it. I just want to quibble with some details.

1. In a steep dive, a vertical force will be a rearward force. The term "drag" is more descriptive then "lift".

2. As the dive begins, the plain's weight is larger than drag and it accelerates. If it accelerates long enough (and if it does not hit the ground and if things do not otherwise change first) it will eventually approach terminal velocity and stop accelerating downward.

3. The seat is accelerating downard almost as fast as the passengers would if they were in free fall. If there is any drag at all, the plane will not be accellerating downward that fast.

Again, these are just quibbles. The key observation is the one you ended with -- we perceive our weight through the force of the ground or the seat on us.

10. Jun 11, 2015

pat devine

Hi. This question appears to be more about what the biological sensation of weightlessness is. For this particular question or better still imagine someone falling freely to the earth, in the absence of any resistance all of your body including internal organs are all falling at the same rate. Hence internally your organs are essentially floating with respect to your chest/abdomen as opposed the normal situation in which they are all sitting on top of each other. The release of this downwards pressure creates the sensation of weightlessness.