Foot pounds-force minute to Joules/second
ft·lbf/min
J/s
Conversion History
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Quick Reference Table (Foot pounds-force minute to Joules/second)
| Foot pounds-force minute (ft·lbf/min) | Joules/second (J/s) |
|---|---|
| 100 | 2.25969658055847 |
| 1,000 | 22.5969658055847 |
| 5,000 | 112.9848290279235 |
| 10,000 | 225.969658055847 |
| 33,000 | 745.6998715842951 |
| 100,000 | 2,259.69658055847 |
| 330,000 | 7,456.998715842951 |
About Foot pounds-force minute (ft·lbf/min)
Foot pounds-force per minute (ft·lbf/min) equals approximately 0.02260 watts. It is used in US mechanical engineering for low-power applications and in the historical definition of horsepower: one horsepower was defined by James Watt as 33,000 ft·lbf/min — the rate at which a horse could lift coal from a mine. This unit is now mostly encountered in legacy engineering references and historical machinery specifications.
One mechanical horsepower = 33,000 ft·lbf/min. A hand-cranked generator might produce 2,000–5,000 ft·lbf/min of mechanical power output.
About Joules/second (J/s)
Joules per second (J/s) is the dimensional expression of power in the SI system, and is exactly equivalent to the watt by definition. While "watt" is the named unit used in practice, J/s appears in physics derivations, dimensional analysis, and engineering calculations where explicit unit tracking is required. Seeing power written as J/s emphasizes the energy-per-time nature of the quantity and connects power directly to the joule and second without introducing a derived unit name.
A 100 W light bulb consumes 100 J/s of electrical energy. A person climbing stairs at moderate pace expends roughly 300 J/s of mechanical power.
Foot pounds-force minute – Frequently Asked Questions
Where does the number 3,960 come from in US pump sizing formulas?
The pump horsepower formula HP = (GPM × Head in ft) / 3,960 hides a chain of unit conversions. Water weighs 8.33 lb per US gallon. Multiplying GPM × Head × 8.33 gives ft·lbf/min. Dividing by 33,000 ft·lbf/min per hp gives horsepower. So 33,000 ÷ 8.33 ≈ 3,960. The number is so ubiquitous in US mechanical engineering that pump designers recognize it on sight, yet few remember the derivation. It breaks down for fluids other than water — multiply by specific gravity for anything denser or lighter.
What real-world tasks produce 33,000 ft·lbf/min?
Lifting 330 lbs (150 kg) at 100 feet per minute — roughly the speed of a slow freight elevator. Or lifting 33 lbs at 1,000 ft/min (a fast dumbwaiter). A human on a bicycle sustainably produces about 5,000–10,000 ft·lbf/min (0.15–0.3 hp). A small outboard boat motor produces about 165,000 ft·lbf/min (5 hp). The unit makes intuitive sense for lifting and hoisting — the original application Watt cared about.
Why use ft·lbf/min instead of ft·lbf/s?
Historical convention and practical timescale. Mine hoists, waterwheels, and early steam engines operated at rates naturally measured per minute — the machinery completed one cycle every few seconds to minutes. Watt himself measured horses per minute because that's how mine work was timed. The per-minute unit also gives larger, more manageable numbers: "33,000 ft·lbf/min" is easier to work with than "550 ft·lbf/s" when you're doing longhand arithmetic in 1780.
How much ft·lbf/min can a human sustain?
A healthy adult can sustain about 4,000–6,000 ft·lbf/min (roughly 90–135 W or 0.12–0.18 hp) of useful mechanical work for hours — think steady cycling or rowing. Short bursts reach 15,000–25,000 ft·lbf/min (0.5–0.75 hp). Elite cyclists sustain 12,000+ ft·lbf/min (0.4 hp) for an hour. By Watt's definition, a horse sustains 33,000 ft·lbf/min, meaning one horse ≈ 5–8 sustained humans. The ancient rule of "ten slaves per horse" wasn't far off.
Is 33,000 ft·lbf/min still used in any modern calculations?
Yes — it's embedded in US pump and fan engineering. The formula for pump horsepower is: HP = (GPM × Head in ft × Specific Gravity) / 3,960, where 3,960 = 33,000 / (8.33 lb/gal). The number 33,000 ft·lbf/min lurks inside every US pump sizing calculation, even if the engineer never writes it explicitly. It also appears in ASME standards for hoists, cranes, and elevators — anywhere lifting power needs to be specified.
Joules/second – Frequently Asked Questions
Why would anyone write joules per second instead of watts?
In dimensional analysis and physics derivations, writing J/s keeps the units transparent — you can see exactly what's being divided and multiplied. If you're calculating power as force × velocity (N·m/s = J/s), keeping it as J/s avoids a mental leap. Students and textbook authors prefer it when teaching the concept of power, because "energy per time" is more intuitive than a named unit. Once you understand it, you switch to watts for brevity.
Is joules per second used in any official standards or regulations?
The SI system officially defines the watt as the named unit for power, with J/s as its definition. In metrology documents and BIPM publications, you'll see W = J/s = kg·m²/s³. Some ISO standards for calorimetry and heat flow measurements express power in J/s to maintain consistency with energy measurements also given in joules. In practice, scientific papers in thermodynamics and physical chemistry often prefer J/s for clarity.
How does expressing power as J/s help in physics problem solving?
It makes unit cancellation visible. If you know a machine delivers 500 J of work over 10 seconds, writing 500 J ÷ 10 s = 50 J/s is a complete, self-checking calculation. Converting immediately to "50 W" obscures the path. In thermodynamics, where you track joules of heat, joules of work, and joules per second of power flow, keeping J/s prevents sign and unit errors that plague students.
What is the relationship between J/s and other compound SI units?
J/s = W = V·A = kg·m²/s³. Each form has its domain: electrical engineers think V·A, mechanical engineers think N·m/s, and physicists think kg·m²/s³. The beauty of SI is that they're all identical. A volt is a J/C, an ampere is C/s, so V·A = J/C × C/s = J/s. This chain of definitions means you can derive any electrical quantity from mass, length, time, and current.
Are there situations where J/s and watts give different numbers?
Never — they are exactly identical by definition, with zero rounding or conversion error. 1 J/s = 1 W, always. This is unlike, say, calories per second vs. watts, where a conversion factor (4.184) introduces potential rounding issues. The equivalence is definitional, not empirical. If someone claims a difference exists, they're confusing joules per second with some other energy-per-time unit like calories per second or BTU per hour.