I have discussed this subject in detail before...
Check out this part of the gravity sticky thread.
Allow me to summarise:
- an accelerometer in your hand while you sit on the sofa reads +9.8ms
-2. This is the result of what is commonly called the
normal contact force of your arse with the sofa. It acts in an upwards direction, hence the '+'.
- an accelerometer in your hand as you stand in the doorway of a plane flying at constant altitude reads +9.8ms
-2, again due to the contact force of you on the plane. The plane maintains altitude by generating lift, essentially pushing on the air to generate an upwards force.
Now you jump out of the plane door...
- the accelerometer initially reads
zero as you step out the door, then as you accelerate
with respect to the air around you, you find that you measure a
steadily increasing positive force resulting from
air resistance.
- when this force equals +9.8ms
-2, you stop accelerating
with respect to the ground and continue at constant velocity, known as
terminal velocity.
- as you hit the ground, the accelerometer records a
rapid increase in positive acceleration, again as a result of the
contact force with the ground, before returning to +9.8ms
-2.
Note that
at no point does the accelerometer measure a
negative force in the form of 'gravity acting downwards'. An accelerometer can never measure gravity
directly, although it can measure it
indirectly as in the contact force of an object stationary on the ground, or moving from one altitude to another.