Pound (force) facts for kids

The pound-force (symbol: lbf) is a special unit that measures force. Think of force as a push or a pull. It's used in some older measurement systems, like the ones used in engineering in English-speaking countries.

It's important not to mix up pound-force with other similar-sounding terms!

• Pound-mass (lb) measures how much "stuff" something has. This is often just called "pound."
• Foot-pound (ft⋅lbf) measures energy, which is the ability to do work.
• Pound-foot (lbf⋅ft) measures torque, which is a twisting force.

What is Pound-Force?

The pound-force is basically the amount of force that gravity pulls on one avoirdupois pound of mass when it's on the surface of Earth.

For a long time, people used this unit for everyday measurements. They didn't worry much about small changes in Earth's gravity. Gravity changes slightly depending on where you are, like at the equator versus the North Pole.

But as science and engineering became more precise, a clearer definition was needed.

How Pound-Force is Defined

Today, pound-force is officially defined using a specific value for gravity. It's the result of multiplying one avoirdupois pound of mass by the standard acceleration due to gravity.

One avoirdupois pound is exactly 0.45359237 kg (1.0000000 lb). The standard acceleration due to gravity is 9.80665 m/s² (32.1740 ft/s²).

When you multiply these two values, you get the exact value for one pound-force.

• One pound-force is equal to about 4.4482216152605 N (1.0000000000000 lb_f) (Newtons).
• The Newton is the standard unit of force in the metric system.

Another way to think about it involves a unit called the slug. A slug is a unit of mass that is much larger than a pound-mass.

• One slug has a mass of about 32.174 pounds.
• One pound-force is the amount of force needed to make a slug speed up at a rate of 1 ft/s².

Converting to Other Units

You can convert pound-force to other units of force. For example, it can be changed into Newtons, which are used in the metric system.

Foot–Pound–Second (FPS) Systems

In some older systems, the word "pound" can be confusing. Sometimes it means a unit of force, and sometimes it means a unit of mass.

In engineering systems, the weight of one pound-mass on Earth is roughly equal to one pound-force. This is handy because it means a one-pound object pulls down with about one pound of force.

However, in these systems, you need an extra number when using Newton's second law (Force = mass × acceleration). This is because the units don't perfectly match up.

The SI (metric) system is simpler. It uses kilograms for mass and meters per second squared for acceleration. This makes calculations straightforward without extra numbers.

Pound of Thrust

The term pound of thrust is another name for pound-force. It's often used when talking about jet engines and rockets, especially in the United States.

For example, the powerful Space Shuttle's Solid Rocket Boosters each produced about 3,300,000 pounds-force (14.7 MN) of thrust. Together, they made about 6,600,000 pounds-force (29.4 MN) of thrust!

See also

In Spanish: Libra-fuerza para niños