Foot-pound (energy) facts for kids

The foot-pound force (often written as ft⋅lbf or ft⋅lb) is a way to measure energy or work. It is used in the U.S. customary and imperial systems.

Imagine you lift something. The foot-pound force tells you how much energy you used. It is the energy needed to move a force of one pound-force (lbf) a distance of one foot. The standard unit for energy in science (the SI unit) is the joule.

What is a Foot-Pound Force?

The foot-pound force measures how much energy is transferred when a force moves an object. For example, if you push something with a force of one pound and it moves one foot, you have done one foot-pound of work. This unit helps engineers and scientists talk about energy in a clear way.

How Foot-Pounds Are Used

The foot-pound is often used in different areas, especially when talking about how much power something has.

Measuring Bullet Energy

In the United States, the foot-pound is commonly used to describe the muzzle energy of a bullet. Muzzle energy is the amount of energy a bullet has right when it leaves the barrel of a gun. A higher foot-pound value means the bullet has more energy and can hit harder.

Understanding Torque (Twisting Force)

The term foot-pound can also describe torque. Torque is a twisting force. Think about tightening a nut with a wrench. The force you apply to the wrench creates torque, which twists the nut. This is often called a "pound-foot" to avoid confusion with energy.

For example, mechanics use foot-pounds to measure how tightly they should fasten screws and nuts on an engine. A car engine's power output can also be measured in foot-pounds of torque.

Energy vs. Torque

Even though both energy and torque use "foot" and "pound," they are different:

• Energy is about moving something in a straight line. It is a scalar quantity, meaning it only has a size (like 10 foot-pounds).
• Torque is about twisting something. It is a vector quantity, meaning it has both a size and a direction (like 10 pound-feet clockwise).

To avoid mixing them up, people sometimes say "foot-pound of energy" or "foot-pound of torque."

Converting Foot-Pounds

It is helpful to know how foot-pounds compare to other units of measurement.

Energy Conversions

One foot-pound force is equal to:

• About 1.356 joules (the standard scientific unit for energy).
• About 13,558,179 ergs (another small unit of energy).
• About 0.001285 British thermal units (BTUs, used for heating).
• About 0.324 calories (used for food energy).

Power Conversions

Power is how fast energy is used or transferred. One foot-pound force per second is equal to:

• About 1.356 watts (the standard scientific unit for power).
• About 0.0018 horsepower (a common unit for engine power).

Other useful power conversions include:

• 1 watt is about 0.738 foot-pounds per second.
• 1 horsepower (mechanical) is equal to 550 foot-pounds per second.

See also

In Spanish: Pie-libra fuerza para niños

• Converting Units
• Pound-foot (torque)
• Units of energy