pound-force square second/foot

What Is pound-force square second/foot?

Pound-force square second per foot is a unit used to measure mass. It belongs to the English engineering system. One pound-force square second per foot is the same mass as one slug. This unit is built from force, time, and distance, based on the law of motion that links force, mass, and acceleration.

Definition

Pound-force square second per foot tells us how hard it is to change the motion of an object when force is measured in pound-force and distance in feet.

More formally:

• Symbol: lbf·s2/ft
• Quantity measured: mass or inertia
• 1 lbf·s²/ft is defined so that a mass of 1 lbf·s²/ft will accelerate at 1 foot per second squared when acted on by a net force of 1 pound-force

In the English engineering system this mass is called one slug. So:

1 pound-force square second per foot = 1 slug

History / Origin

This unit comes from the need to keep everyday force units in pounds while still using Newtons second law of motion, which says:

Force = Mass × Acceleration

In the metric system this law uses newtons for force, kilograms for mass, and meters per second squared for acceleration. In the English engineering system engineers wanted to use:

• pound-force (lbf) for force
• foot (ft) for length
• second (s) for time

To make the law of motion work with these units, they built a mass unit from them. Rearranging the law gives:

Mass = Force × Time2 ÷ Distance

If we plug in the English units, we get lbf·s²/ft. That expression became the formal way to show mass in that system. The special name slug was later given to this mass, but the unit expression pound-force square second per foot is still used in careful technical writing.

Symbol & Abbreviation

The most common symbols and short forms for pound-force square second per foot are:

• lbf·s2/ft standard symbol
• lbf s2/ft same unit, dot left out
• lbf·s2/ft sometimes used in textbooks

All of these symbols mean the same thing. They show that the unit is made from:

• lbf pound-force
• s² second squared, or seconds times seconds
• ft foot

Current Use Around the World

Pound-force square second per foot is not used much in everyday life. You will mainly see it in:

• engineering books that use the English engineering system
• older physics and mechanics texts from the United States
• some aerospace and mechanical engineering calculations that keep force in pound-force

Most of the world now uses the metric system with kilograms for mass. Even in countries that still use pounds in daily life, like the United States, scientists and many engineers prefer kilograms and newtons. Because of this the unit pound-force square second per foot is becoming less common, but it is still important to understand when reading older work or special engineering documents.

Example Conversions

Because one pound-force square second per foot equals one slug, we can use known conversion factors for the slug.

Key relationships

• 1 lbf·s²/ft = 1 slug
• 1 slug ≈ 14.5939 kilograms (kg)
• 1 slug ≈ 32.174 pounds mass (lbm)

Example 1 convert 1 lbf·s²/ft to kilograms

Start with the unit:

1 lbf·s²/ft = 1 slug

Now convert slugs to kilograms:

1 slug ≈ 14.5939 kg

Answer: 1 lbf·s²/ft ≈ 14.59 kg

Example 2 convert 5 lbf·s²/ft to kilograms

5 lbf·s²/ft = 5 slugs

Now multiply:

5 × 14.5939 kg ≈ 72.9695 kg

Answer: 5 lbf·s²/ft ≈ 73.0 kg (rounded)

Example 3 convert 2 lbf·s²/ft to pounds mass

2 lbf·s²/ft = 2 slugs

1 slug ≈ 32.174 lbm

2 × 32.174 lbm ≈ 64.348 lbm

Answer: 2 lbf·s²/ft ≈ 64.35 lbm

Related Units

Pound-force square second per foot is closely linked to several other mass units:

• Slug the named mass unit that is exactly equal to 1 lbf·s²/ft
• Kilogram (kg) the base unit of mass in the metric system, used worldwide in science
• Pound-mass (lbm) a mass unit used in some US engineering and everyday life
• Newton second squared per meter (N·s²/m) a way to write mass using metric force and length units, which is equal to the kilogram

In simple terms:

• 1 lbf·s²/ft = 1 slug
• 1 slug ≈ 14.5939 kg ≈ 32.174 lbm

FAQs

What does pound-force square second per foot measure?

It measures mass, which tells us how hard it is to speed up, slow down, or change the direction of an object when force is measured in pound-force.

Is pound-force square second per foot the same as a slug?

Yes. By definition, 1 pound-force square second per foot is equal to 1 slug. The slug is just a shorter name for the same amount of mass.

Why is the unit name so long?

The name shows how the unit is built from other units. It is made from pound-force, seconds, and feet, following the rule Mass = Force × Time² ÷ Distance.

Do people still use this unit today?

It is mostly used in older textbooks and in some engineering fields that still work with pound-force and feet. In most new work, engineers prefer kilograms and newtons.

How do I convert pound-force square second per foot to kilograms?

First change it to slugs, then to kilograms. Since 1 lbf·s²/ft = 1 slug and 1 slug ≈ 14.5939 kg, you multiply the number of lbf·s²/ft by 14.5939 to get kilograms.

Is pound-force square second per foot a base unit or a derived unit?

It is a derived unit. It is built from the base units pound-force, second, and foot, combined using the law of motion in the English engineering system.