Force from Acceleration Only?

We all know 

  F = m * A

If I have only the knowledge from an accelerometer, is it possible to
determine force (F)? 

Sure seems to me the answer is no, but I thought I'd check with the
experts here.

Yes, of course I know that you could get an _estimate_ of force if you
had an estimate of mass. Assume no such estimate exists.
-- 
Randy Yates
Digital Signal Labs
http://www.digitalsignallabs.com

Den torsdag den 1. maj 2014 23.55.06 UTC+2 skrev Randy Yates:
> We all know 
> 
> 
> 
>   F = m * A
> 
> 
> 
> If I have only the knowledge from an accelerometer, is it possible to
> 
> determine force (F)? 
> 
> 
> 
> Sure seems to me the answer is no, but I thought I'd check with the
> 
> experts here.
> 
> 
> 
> Yes, of course I know that you could get an _estimate_ of force if you
> 
> had an estimate of mass. Assume no such estimate exists.
> 

odd question, how could you?

if you sit still your acceleration will be ~9.82m/s^2 
doesn't give me any clue to how much you weight or how much force 
you put on the chair


-Lasse

On Thu, 01 May 2014 17:55:06 -0400, Randy Yates wrote:

> We all know
> 
>   F = m * A
> 
> If I have only the knowledge from an accelerometer, is it possible to
> determine force (F)?
> 
> Sure seems to me the answer is no, but I thought I'd check with the
> experts here.
> 
> Yes, of course I know that you could get an _estimate_ of force if you
> had an estimate of mass. Assume no such estimate exists.

Your intuition is telling you something.  If you listen harder, it'll 
tell you you're trying to solve one equation in two (actually three) 
unknowns.

The unknowns are the mass, the force, and the acceleration of the frame 
(thanks Lasse, for pointing that out).

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Is the accelerometer mounted In a system that has a resonant frequency ? If so you you might be able to detect the resonance with a bit of dsp and infer the mass. 

Bob

On Thursday, May 1, 2014 5:55:06 PM UTC-4, Randy Yates wrote:
> We all know 
> 
> 
> 
>   F = m * A
> 
> 
> 
> If I have only the knowledge from an accelerometer, is it possible to
> 
> determine force (F)? 
> 
> 
> 
> Sure seems to me the answer is no, but I thought I'd check with the
> 
> experts here.
> 
> 
> 
> Yes, of course I know that you could get an _estimate_ of force if you
> 
> had an estimate of mass. Assume no such estimate exists.
> 
> -- 
> 
> Randy Yates
> 
> Digital Signal Labs
> 
> http://www.digitalsignallabs.com

Randy,

F=m*a is actually a simplified form of Newton's 2nd law.

Actually it is 

F = d/dt (momentum) = d/dt (m(t)*v(t)) = m'*v + m*a  (v = velocity)

If your mass is constant it reduces to the familiar m*a but if your mass is changing, (such as a rocket consuming its fuel) you will need the actual mass function. So if you have an accelerometer on a rocket, can you find the engine's thrust (force) without making some assumptions about something being constant?

Clay

clay@claysturner.com wrote:

(snip)

> F=m*a is actually a simplified form of Newton's 2nd law.
 
> Actually it is 
 
> F = d/dt (momentum) = d/dt (m(t)*v(t)) = m'*v + m*a  (v = velocity)
 
> If your mass is constant it reduces to the familiar m*a but 
> if your mass is changing, (such as a rocket consuming its 
> fuel) you will need the actual mass function. 

Reminds me that we haven't heard from Jerry recently.

Some years ago, he reminded us that inductors are not V=L dI/dt,
but V=d(LI)/dt, or L dI/dt + I dL/dt, and that in the case of
solenoids with moving iron core, it has to be included.

-- glen

On 5/2/14 1:45 PM, glen herrmannsfeldt wrote:
> clay@claysturner.com wrote:
>
> (snip)
>
>> F=m*a is actually a simplified form of Newton's 2nd law.
>
>> Actually it is
>
>> F = d/dt (momentum) = d/dt (m(t)*v(t)) = m'*v + m*a  (v = velocity)
>
>> If your mass is constant it reduces to the familiar m*a but
>> if your mass is changing, (such as a rocket consuming its
>> fuel) you will need the actual mass function.
>
> Reminds me that we haven't heard from Jerry recently.
>
> Some years ago, he reminded us that inductors are not V=L dI/dt,
> but V=d(LI)/dt, or L dI/dt + I dL/dt, and that in the case of
> solenoids with moving iron core, it has to be included.
>


in the old Wurlitzer Model 200 electric piano, they have 64 tuned tines 
(one for each key on the keyboard) that are whacked by a mechanical 
linkage that is similar to a regular piano.  the tines are steel and 
bolted tightly against an solid steel block that is grounded.  sitting 
very closely to the tines is a plate that's lit up to 130 V DC.

how do you think this electrostatic pickup works?


-- 

r b-j                  rbj@audioimagination.com

"Imagination is more important than knowledge."

On Fri, 02 May 2014 17:45:56 +0000, glen herrmannsfeldt wrote:

> clay@claysturner.com wrote:
> 
> (snip)
> 
>> F=m*a is actually a simplified form of Newton's 2nd law.
>  
>> Actually it is
>  
>> F = d/dt (momentum) = d/dt (m(t)*v(t)) = m'*v + m*a  (v = velocity)
>  
>> If your mass is constant it reduces to the familiar m*a but if your
>> mass is changing, (such as a rocket consuming its fuel) you will need
>> the actual mass function.
> 
> Reminds me that we haven't heard from Jerry recently.
> 
> Some years ago, he reminded us that inductors are not V=L dI/dt,
> but V=d(LI)/dt, or L dI/dt + I dL/dt, and that in the case of solenoids
> with moving iron core, it has to be included.
> 
> -- glen

Ditto capacitors, for that matter.  You can turn that around:

V = C * q, where q is charge, and q = {integrate} I dt, and

I = L * phi (I think it's phi), where phi is the total magnetic flux or 
some such, and is equal to phi = {integrate} V dt.

Keeping that in mind, you can solve for the V = d/dt (L * I), or I = d/dt 
(C * V).

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

On 2014-05-02 13:01, radams2000@gmail.com wrote:
> Is the accelerometer mounted In a system that has a resonant frequency ? If so you you might be able to detect the resonance with a bit of dsp and infer the mass. 

Sorry, but are you sure of that?

I mean, does 1Kg, 1m metal bar have the same
resonance frequency of 1Kg, 2m metal bar?

Thanks,

bye,

> Bob
> 


-- 

piergiorgio

On Saturday, May 3, 2014 2:18:26 AM UTC+12, cl...@claysturner.com wrote:
> On Thursday, May 1, 2014 5:55:06 PM UTC-4, Randy Yates wrote:
> 
> > We all know 
> 
> > 
> 
> > 
> 
> > 
> 
> >   F = m * A
> 
> > 
> 
> > 
> 
> > 
> 
> > If I have only the knowledge from an accelerometer, is it possible to
> 
> > 
> 
> > determine force (F)? 
> 
> > 
> 
> > 
> 
> > 
> 
> > Sure seems to me the answer is no, but I thought I'd check with the
> 
> > 
> 
> > experts here.
> 
> > 
> 
> > 
> 
> > 
> 
> > Yes, of course I know that you could get an _estimate_ of force if you
> 
> > 
> 
> > had an estimate of mass. Assume no such estimate exists.
> 
> > 
> 
> > -- 
> 
> > 
> 
> > Randy Yates
> 
> > 
> 
> > Digital Signal Labs
> 
> > 
> 
> > http://www.digitalsignallabs.com
> 
> 
> 
> Randy,
> 
> 
> 
> F=m*a is actually a simplified form of Newton's 2nd law.
> 
> 
> 
> Actually it is 
> 
> 
> 
> F = d/dt (momentum) = d/dt (m(t)*v(t)) = m'*v + m*a  (v = velocity)
> 
> 
> 
> If your mass is constant it reduces to the familiar m*a but if your mass is changing, (such as a rocket consuming its fuel) you will need the actual mass function. So if you have an accelerometer on a rocket, can you find the engine's thrust (force) without making some assumptions about something being constant?
> 
> 
> 
> Clay

Lex II: Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur.

Law II: The alteration of motion is ever proportional to the motive force impress'd; and is made in the direction of the right line in which that force is impress'd.

. 
Newton did not write down the equation F=ma - it was first written down by Euler several decades later. Nor did Newton provide in his published work either a definition or a formula for his "quantity of motion", which is commonly interpreted to be his term for "momentum".


Jean Buridan (in Latin, Johannes Buridanus) (c. 1300 - after 1358) was a French priest who sowed the seeds of the Copernican revolution in Europe. He developed the concept of impetus, (momentum)