[annie123 (OP)]

I have seen people in planes which mimic the no gravity scenario when the plane is on the ground - how is that done? And also BCox showed himself in a plane descending and the effect of no gravity was produced but he didn't explain what was happening.
How do either of these work? And if a plane is crashing - descending very fast - would hte people in it be experiencing the same?

[evan_au]

Brian Cox showed himself in a plane descending and the effect of no gravity was produced but he didn't explain what was happening.

If an aeroplane follows a careful parabolic trajectory (like a ball thrown up in the air), anything inside the plane will be following the same path as the plane (like a ball thrown up in the air), and they will be weightless (relative to the plane).

This method gives about 30 seconds of weightlessness, followed by 2 minutes of increased weight as the plane recovers from its dive, and positions itself for the next parabolic "throw". This produces an extreme form of airsickness, earning the nickname "vomit comet".

The weightless period does not just happen on the descent - it also occurs on the rising segment of the parabola, too.
See: https://en.wikipedia.org/wiki/Reduced-gravity_aircraft

Space agencies have used these for astronaut training, and I recently saw Heston Blumenthal testing his prototype space food in a commercial version.
http://www.irishexaminer.com/examviral/celeb-life/heston-blumenthal-hosted-the-most-stressful-dinner-party-ever--for-tim-peake-on-board-the-iss-387103.html

I have seen people in planes which mimic the no gravity scenario when the plane is on the ground - how is that done?

I think this is a matter of the film crew taking a picture of the plane before or after the parabolic ride; to take a picture of the plane in flight requires a camera on a chase jet - and a very sick cameraman in a confined space. Vomiting into your oxygen mask is not a good idea.

[alancalverd]

And if a plane is crashing - descending very fast - would the people in it be experiencing the same?

Free fall, where the plane and its contents are all accelerating at g (9.81 m/s^2 or thereabouts) is unusual and generally of short duration. The usual cause of a crash is a spin, in which one wing is still flying whilst the other is stalled, or a spiral dive, with both wings generating lift but in the wrong direction. In a spin, the rate of descent usually stabilises well below the terminal speed of the plane and the vertical forces return to near normal. A spiral dive can reach terminal speed but in this attitude the lift is acting towards the axis of the spiral and you experience a centrifugal force that can exceed g. A sustained vertical power dive is unusual, but you can be accelerating downwards at more than g, so free objects inside the aircraft get pushed "backwards" and experience a similar g force to a full-power climb or level acceleration.

Survival when flying in cloud depends on being able to ignore the g experience and concentrate on the gyro instruments which will give you true pitch and rotation information. And just to make it interesting, your semicircular canals tend to adapt to sustained g so that, on recovering from e.g. a steep turn, you think you are actually turning in the opposite direction, and your instinct is to revert back to the original turn because it feels more like level flight!   

[McQueen]

I have seen people in planes which mimic the no gravity scenario when the plane is on the ground - how is that done? And also BCox showed himself in a plane descending and the effect of no gravity was produced but he didn't explain what was happening.
How do either of these work? And if a plane is crashing - descending very fast - would hte people in it be experiencing the same?

The short answer is that it can't be done on the ground, unless an apparatus similar in size to the LHC were used.  If TV props are allowed then a very large and well insulated fan, could create the same kind of effect but it would not be weightlessness, more like a simulation for free fall parachuting.  To the last question no-one would be alive to answer the question, except perhaps if the plane started very high up.