Terminal velocity when falling through the atmosphere

[JimT]

Dear all,

I have been working my way through the Oxford University physics test sample paper http://www.physics.ox.ac.uk/admissions/combined-sample.pdf" and I have come across this question:

A sky-diver jumps out of an aeroplane. Which of the following statements is true after she reaches terminal velocity?
A The force of air resistance is the same size as her weight.
B The force of air resistance is larger than her weight.
C The force of air resistance is smaller than her weight.
D She begins to slow down.

Now my initial answer would be A however, after thinking about it would I be correct in believing that the answer would be in fact D as air density decreases with increase in altitude? The net force, if the sky-diver stayed at a constant terminal velocity, would steadily become greater in magnitude (after reaching initial terminal velocity) in the upwards direction as a result of increased air resistance due to an increase in air density. To resolve this net difference, the sky-diver's velocity would decrease thus lowering drag until the net force=0?

Would that be correct? As far as I can see, both A and D are true.

Best wishes,
James

 

[arildno]

It is true enough that the density of air changes somewhat with altitude, yet the PREMISE of this exercise is that she HAS reached terminal velocity, which is therefore terminal, i.e, final and CONSTANT.

What has confused you is rather the question "Is the premise justifiable in the first place?"

As it happens, it IS justified, and your uncertainty is misplaced, but far more important is that it is an irrelevant concern to the given question.

 

[JimT]

Okay, thanks that's super.