# For Doc, rates of change of acceleration

1. Mar 8, 2004

### 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 "&Delta;a" 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.

2. Mar 8, 2004

### Integral

Staff Emeritus
You are correct.

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

$$\frac m {s^3}$$

3. Mar 8, 2004

### Staff: Mentor

As Integral states, the rate of change of acceleration is called "jerk".
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 &Delta;a; rate of change is &Delta;a/&Delta;t.

4. Mar 8, 2004

### Micky

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

5. Mar 8, 2004

### Micky

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 ???????