For Doc, rates of change of acceleration

[Micky]

Sometime ago you very kindly advised me regarding units for rates of change of acceleration.

I still don't fully understand how "a" and "Δa" can have the same units, "g". If acceleration is measured in terms of "g", shouldn't the rate of change of acceleration be measured in terms of g/s?

Thanks.

[Integral]

You are correct.

rate of change of acceleration, also called jerk, has units

\frac m {s^3}

[Doc Al]

Originally posted by Micky
Sometime ago you very kindly advised me regarding units for rates of change of acceleration.
As Integral states, the rate of change of acceleration is called "jerk".
I still don't fully understand how "a" and "Δa" can have the same units, "g". If acceleration is measured in terms of "g", shouldn't the rate of change of acceleration be measured in terms of g/s?

You are confusing change with rate of change. The units of a change in any quantity will be the same units as the quantity.

Example: Your weight goes from 165 lbs to 170 lbs. The change is 5 lbs, same units as your weight.

If the change took place over 1 month, the (average) rate of change of your weight would be: 5 lbs/month. Different quantity, different units.

Similary, the CHANGE of acceleration will have units of m/s2 (you can measure in terms of "g" if you want), but the RATE of change of acceleration would be in units of m/s3 (you can use g/sec, if you dare).

Bottom line: Change is Δa; rate of change is Δa/Δt.

[Micky]

Thanks Integral, can jerk also be expressed as g/s?

[Micky]

I've had another look at the original thread here (http://www.physicsforums.com/showthread.php?s=&threadid=13836)

I think I see now how Doc's answer is correct for my method of measuring rates of change of acceleration. I'm comparing two consecutive measurements of acceleration (units = "g"), therefore the answer = change in acceleration over the time period and must be in "g" .. I think. This is fine for the current project which is comparing rates of change of accel.

However, at some stage I will need to express the rate of change of acceleration in units of m/s/s/s. So if the measurement period is, say, 0.01s then I need to multiply the change in acceleration over this period by 100.

I suspect that the calculation should be {measured accel value in units of g}*9.8*100 which should then give me the rate of change of acceleration measured in m/s/s/s ???????

Is there a thread with info about inserting symbols into posts?

[Wooh]

Originally posted by Micky
I've had another look at the original thread here (http://www.physicsforums.com/showthread.php?s=&threadid=13836)

I think I see now how Doc's answer is correct for my method of measuring rates of change of acceleration. I'm comparing two consecutive measurements of acceleration (units = "g"), therefore the answer = change in acceleration over the time period and must be in "g" .. I think. This is fine for the current project which is comparing rates of change of accel.

However, at some stage I will need to express the rate of change of acceleration in units of m/s/s/s. So if the measurement period is, say, 0.01s then I need to multiply the change in acceleration over this period by 100.

I suspect that the calculation should be {measured accel value in units of g}*9.8*100 which should then give me the rate of change of acceleration measured in m/s/s/s ???????

Is there a thread with info about inserting symbols into posts?
the LaTeX thread.