Momentum Converter
Convert between different momentum units.
About the Momentum Converter
The Momentum Converter turns a momentum written in one unit into the same momentum written in another. Momentum is mass multiplied by velocity, so its units are always a mass unit times a speed unit: kilogram-metres per second, newton-seconds, or pound-feet per second. Type a number, pick the two units, and the answer updates as you type.
How to convert kg·m/s to lb·ft/s
One pound-foot per second is exactly 0.138254954376 kg·m/s. The factor is exact because both of its parts are defined exactly: a pound is 0.45359237 kg and a foot is 0.3048 m, and 0.45359237 × 0.3048 = 0.138254954376.
Worked example. A 1,500 kg car at 20 m/s carries 30,000 kg·m/s of momentum. In imperial units that is 30,000 ÷ 0.138254954376 = 217,000 lb·ft/s.
kg·m/s to lb·ft/s conversion table
| kg·m/s | lb·ft/s |
|---|---|
| 1 | 7.233014 |
| 2 | 14.46603 |
| 3 | 21.69904 |
| 5 | 36.16507 |
| 10 | 72.33014 |
| 20 | 144.6603 |
| 50 | 361.6507 |
| 100 | 723.3014 |
| 250 | 1,808.253 |
| 500 | 3,616.507 |
| 1,000 | 7,233.014 |
Why newton-seconds and kg·m/s are the same
A newton is one kg·m/s², so a newton-second is one kg·m/s. The two names describe the same quantity from two directions: the impulse delivered to a body, and the momentum the body carries away. This converter gives them a factor of exactly 1, which is not a rounding — it is an identity.
Real-world values
- A 1,500 kg car at 20 m/s (72 km/h): 30,000 kg·m/s.
- A 0.145 kg baseball at 40 m/s: 5.8 kg·m/s.
- A cyclist and bike together, 85 kg at 10 m/s: 850 kg·m/s.
- A 9 mm bullet, 8 g at 360 m/s: 2.88 kg·m/s.
- A loaded 40 t truck at 25 m/s: 1,000,000 kg·m/s.
- A rocket stage producing 200 kN for 120 s delivers 24,000,000 N·s of impulse.
What is momentum?
Momentum is the product of a body's mass and its velocity, written . It is a vector, so it has a direction as well as a size. Its importance comes from conservation: in a closed system the total momentum before an event equals the total afterwards. That is why collision analysis, rocket propulsion and particle physics are all worked in momentum rather than in speed.
Impulse: how momentum changes
The change in a body's momentum equals the force applied multiplied by the time it acts, which is exactly why the newton-second exists as a unit. A 10 N force acting for 3 s changes momentum by 30 kg·m/s, whatever the mass of the body. Airbags and crumple zones work on this arithmetic: they cannot reduce the momentum a crash has to absorb, only stretch the time over which it is absorbed, which lowers the peak force.
Units this converter supports
Three: kilogram-metre per second (kg·m/s), newton-second (N·s) and pound-foot per second (lb·ft/s). kg·m/s is the base unit, and the other two convert through it, so the table above and the converter agree exactly.
Frequently asked questions
Is a newton-second the same as a kg·m/s?
Yes, exactly the same. A newton is kg·m/s², so a newton-second is kg·m/s. The two names describe the same quantity from different directions: impulse applied, and momentum carried. The converter treats them as equal, with a factor of 1.
How do I convert lb·ft/s to kg·m/s?
Multiply by 0.138254954376. That factor is exact, because it is the pound in kilograms (0.45359237) multiplied by the foot in metres (0.3048). Divide by it to go the other way.
How do I work out momentum in the first place?
Multiply mass by velocity: p = m·v. A 1,500 kg car at 20 m/s carries 30,000 kg·m/s. Momentum points the same way the velocity does, and the total is conserved in a collision, which is why it is the quantity crash analysis works in.
Which units does this converter support?
Three: kilogram-metre per second (kg·m/s), newton-second (N·s) and pound-foot per second (lb·ft/s). kg·m/s is the base unit, and every conversion is computed through it.
Related converters
- Mass Converter — kilograms, pounds, stones and tons.
- Force Converter — newtons and pound-force.
- Acceleration Converter — m/s², g-force and ft/s².