 # Determining accleration due to gravity

I have an accelerometer connected to an Arduino Uno that reads out 1 G when laying flat on a table. When it is in free fall that changes to 0g. Does anyone know the necessary math to convert this to acceleration so that when it is in free fall acceleration is 9.8m/s^s and when it is at rest acceleration is 0m/s^2? Thanks.

Otherwise there is nothing to go on besides speculating about something like this: http://arduino.cc/en/Tutorial/ADXL3xx

This is the accelerometer https://www.sparkfun.com/products/9652 with the same wiring and code as shown in this tutorial http://www.geeetech.com/wiki/index.php/MMA7361_Triple_Axis_Accelerometer_Breakout . It outputs the g forces along each axis and I have taken the square root of(x^2+y^2+z^2) to determine the magnitude. Hope this helps and thanks for your help.

Never mind I figured it out. Sorry for the inconvenience.

What was the solution?

Since gravity is always constant I have to subtract that out of the values. So at rest when it said g=1 then subtract 1 *9.8m/s^2 = 0 and when it is free falling with a reading of 0 then subtract 1 * 9.8m/s^2 = -9.8m/s^2

Ben1234: Since gravity is always constant I have to subtract that out of the values. So at rest when it said g=1 then subtract 1 *9.8m/s^2 = 0 and when it is free falling with a reading of 0 then subtract 1 * 9.8m/s^2 = -9.8m/s^2

Technically the force of Earth's gravity is not constant from place to place and it can even slowly vary a bit over time. However, for your purposes this is should be good enough. ;)

Far-seeker:

Ben1234: Since gravity is always constant I have to subtract that out of the values. So at rest when it said g=1 then subtract 1 *9.8m/s^2 = 0 and when it is free falling with a reading of 0 then subtract 1 * 9.8m/s^2 = -9.8m/s^2

Technically the force of Earth's gravity is not constant from place to place and it can even slowly vary a bit over time. However, for your purposes this is should be good enough. ;)

Technically technically, the force of gravity depends on mass and is called weight. The acceleration due to gravity is the one that is relatively constant. Although you are right it does vary slightly from place to place and time to time, but I doubt the OP is going to measure that with his accelerometer.

If the accelerometer is accurate enough, wouldn't it be able to detect these changes and output an acceleration? Then you could use that as a calibration to find the percentage of gravity compared to standard (9.8N/kg) at that location.

packocrayons: If the accelerometer is accurate enough, wouldn't it be able to detect these changes and output an acceleration? Then you could use that as a calibration to find the percentage of gravity compared to standard (9.8N/kg) at that location.

Theoretically yes. However, I doubt the accelerometers that most of us can afford to use in our projects would be capable that level of both accuracy and precision. I was really just engaging in some harmless snark.

Of course the real mind-bending thing is to realize that the accelerometer is correct, an object in free fall isn’t accelerating Depends how long it's been in freefall.

an object in free fall isn't accelerating

So why is its velocity increasing? That bloke that jumped from the balloon, he was free falling, right? And after 40 seconds, he was doing 800mph or so.

An object in "free fall", certainly IS accelerating. For objects that jump out of aircraft or fall off the edge of tables, anyway. If the object is equiped with an accelerometer, it detects no acceleration because the acceleration it is actually undergoing offsets the deflection of the sensor element which normally occurs because of gravity. That is why an accelerometer reads 0 when it is in free fall. It doesn't mean that it is not accelerating, though.

(9.8N/kg)

Interesting units for acceleration there…

I agree it’s correct, since F=ma or a=F/m, but I’ve never seen acceleration expressed in any units other than displacement / time2. It’s a useful way of looking at it, since what we’re interested in as engineers calculating stress is the force exerted on a mass of certain size.

Hmmmm

They are the same unit if you break N down and look at it. N/kg is considered gravitational force while m/s2 is an acceleration, but in reality they are the same thing and both work in any formula.

Yes, I understand the maths… just saying I’d never seen acceleration expressed in those units before.

just saying I’d never seen acceleration expressed in those units before.

You have, you just haven’t done the algebra before

N/kg = (kg m s-2) / kg = (kg m s-2) / kg = m s-2

You have, you just haven't done the algebra before

No, I haven't seen acceleration expressed in those units before. By that I mean, nobody has ever said to me, nor have I ever read, that acceleration comes in N/kg....

I do understand the units' equivalency, and indeed have understood this stuff since about 1971.... I doubt if I'd have graduated as a civil engineer without having understood the relationship between force, mass and acceleration... 8)

packocrayons:
They are the same unit if you break N down and look at it. N/kg is considered gravitational force while m/s2 is an acceleration, but in reality they are the same thing and both work in any formula.

A kilo of mass experiences 9.8 Newton of weight. That was part of my middle school physics class. In high school when I learned F=ma, then a was given in m/s/s.

To correct age-old problem of engineers don’t know physics, I have to clarify that accelerometer is a wrong term. The measured quantity is actually force. Imagine Spring scale and weight on scale. You will read force when the scale is at rest. You will NOT read force when you drop the scale and weight together. The real term should be something like a micromachined capacitive force sensor that outputs acceleration units.