Poundals to newtons: the unit that followed its own definition
Two absolute units, one idea
One poundal is the force that accelerates a one-pound mass at one foot per second squared, and that is the entire definition. Nothing about the strength of gravity appears in it. The newton is the same construction carried out in metric: one kilogram, one metre, one second. So this is less a unit swap than a meeting between two absolute units, and the factor joining them is arithmetic performed on the pound and the foot.
The imperial newton, built from first principles
Put the two definitions side by side and the resemblance is plain. A poundal accelerates 0.45359237 kilogram at 0.3048 metre per second squared, which multiplies out to 0.138254954376 newton. A newton accelerates a whole kilogram by a full metre per second squared. Because neither definition mentions a planet's pull, both units mean the same thing in orbit, on Mars, or in a laboratory where g is an inconvenience rather than a constant.
Poundals to newtons at a glance
Figures either side of the everyday range, from the effort a small mechanism supplies to the thrust of a modest rocket motor.
| Poundal (pdl) | Newton (N) |
|---|---|
| 1 | 0.138254954376 |
| 10 | 1.38254954376 |
| 25 | 3.4563738594 |
| 50 | 6.9127477188 |
| 100 | 13.8254954376 |
| 1000 | 138.254954376 |
Why a principled unit lost anyway
The poundal was internally consistent, and consistency turned out not to be enough. British engineering had already settled on pound-force, where a pound of mass weighs a pound of force at sea level and the number on the scale equals the number in the sum. That convenience beat the purity of the absolute definition. The newton then arrived with the metric system and did the same job for everyone else, leaving the poundal in textbooks and a few specialist corners.
A factor worth carrying in your head
Roughly seven poundals make a newton; precisely, one poundal is 0.138255 newton. To check a result, reverse it — divide the newtons by 0.138255 and the poundals you began with should reappear. When the answer drifts by a factor of ten, the culprit is almost always a decimal point rather than a wrong constant.
Frequently asked questions
What exactly is a poundal?
It is the force that gives a one-pound mass an acceleration of one foot per second squared. The construction uses only the pound and the foot, so the unit owes nothing to gravity at any particular place — a deliberate choice made by the British Association for the Advancement of Science in the 1870s.
Where does the factor 0.138255 come from?
From the two definitions multiplied together. A poundal accelerates 0.45359237 kilogram at 0.3048 metre per second squared, and those two numbers give 0.138254954376 newton. The published factor is that value rounded to six decimal places.
Is the poundal used anywhere today?
Rarely in engineering, occasionally in older British and American textbooks, and in aerodynamics writing where the slug-poundal-second scheme is kept internally consistent. Most modern work is done in newtons, so a poundal figure is usually something being read rather than something being written.
How does the poundal relate to the slug?
The slug is the matching mass unit in the same absolute scheme: one poundal acting on one slug produces one foot per second squared. The scheme is coherent but unfamiliar, because it replaces the pound of mass with a unit about thirty-two times heavier.
Why did the newton displace a unit built this carefully?
Because the newton arrived with a complete metric system and an international standards organisation behind it, while the poundal was competing with pound-force inside its own tradition. A sound definition does not win by itself; adoption does.