# MEMS accelerometer in "free-fall"

Hey everybody,

I've seen on my MEMS accelerometer that it measures an acceleration of roughly 0 during a free-fall in the atmosphere and read that it should gradually highers to 1g upwards until terminal velocity is reached.

Does this mean it actually only measures the acceleration due to drag ?
I wanted to use this as a tool to determine the drag coefficient of a model rocket and possibly the rocket's current airspeed during a ballistic flight using a formula that gives the airspeed corresponding to a certain drag acceleration.

I'm not sure however that it's what the accelerometer measures and if it's the only thing it measures during a free-fall...
Thanks for any replies !

Does this mean it actually only measures the acceleration due to drag ?

It will measure any acceleration to the rocket body, including buffeting from air turbulence, separation of components (like firing out a parachute), etc.

You can think of the sensing element as a mass on a spring, measuring the stretch of the spring. In free fall, gravity acts equally on the mass, spring and supporting framework, and the spring doesn't stretch unless other forces are present.

Elemyr:
Hey everybody,

I've seen on my MEMS accelerometer that it measures an acceleration of roughly 0 during a free-fall in the atmosphere and read that it should gradually highers to 1g upwards until terminal velocity is reached.

Does this mean it actually only measures the acceleration due to drag ?

No it reads acceleration, pure and simple. This is general relativity - weight is the force the earth exerts on you to prevent you free-falling, its acts to accelerate you upwards (but you don't move because the curvature of space exactly counters this) - or put another way the curvature of space defines what free-fall is. When you reach terminal velocity you are no longer in free-fall, the drag force is the same as the weight force the ground would exert.

Basically general relativity is the realization that acceleration due to gravity is the same thing as inertial acceleration - no device can tell the difference without looking outside at the surroundings. Thus a MEMS accelerometer sees itself accelerating upwards at 9.8m/s^2 when its sat "stationary" on the desk...