Understanding the Conversion: Pounds to Kilograms Explained

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So, if one is using English units, and g is taken to be exactly 32.2 feet/second2, then k=1/gc so that\mathbf F = \frac{m\mathbf a}{gc}In summary, the U.S. Customary System has two different units for pounds - one for mass and one for force. This can lead to confusion, as they are not equivalent. The pound force is a measure of force, while the pound mass is a measure of mass. However, they can be converted into each other, although they are only equal at sea-level and under the same gravitational acceleration. The constant "gc" is used in equations to relate the two units, and it is the
  • #1
shamrock5585
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ok i understand how the conversions work and everything... 1 kg = 2.2 pounds... i was just thinking about this for a while and its kinda eating at me because i can't put it into good words.

why is it that when we measure something in the U.S. it is in pounds and we say that is the force it has from gravity not its mass but then if we measure something in kilograms it is mass... so they are different types of units but yet they can be converted into each other?
 
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  • #2
There are two different "pounds"; one is a mass and one is a force. When you perform the conversion you are converting a mass in kg into a mass in lb. The pound force is a measure of force, and is different, but related, to the pound: see here.
 
  • #3
shamrock5585 said:
ok i understand how the conversions work and everything... 1 kg = 2.2 pounds... i was just thinking about this for a while and its kinda eating at me because i can't put it into good words.

why is it that when we measure something in the U.S. it is in pounds and we say that is the force it has from gravity not its mass but then if we measure something in kilograms it is mass... so they are different types of units but yet they can be converted into each other?

Because kg and kgf are mistakenly taken to be equivalent, as are lbm and lbf. They are not equivalent. They are coincidentally equal to the same value at sea-level, i.e. 1 lbm = 1 lbf only under the same gravitational acceleration.

This is a major point of confusion in the US Customary System since the common form of lbf, which is meant to represent weight not mass, is written as lb instead of the appropriate lbf. Hence the confusion.

CS
 
  • #4
ok so 1 kg is equal to 2.2 pound-mass and 1 Newton is equal to .22 pound-force? I guess I am just more familiar with pounds as force and slugs as mass... thanks for your input


ps... if i weigh something on a scale do i consider that to be pounds-force or mass?
 
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  • #5
Although, obviously, scales are actually measuring the weight (the downward force on the scale) they are calibrated to display the mass (they are basically calibrated so that the amount is divided by 9.81 or whatever).
 
  • #6
sweet... thanks for clearing up that confusion
 
  • #7
shamrock5585 said:
ok so 1 kg is equal to 2.2 pound-mass and 1 Newton is equal to .22 pound-force?
Correct.
I guess I am just more familiar with pounds as force and slugs as mass
Force expressed in pounds-force and mass expressed in slugs let's you use Newton's second law in its canonical form, [itex]F=ma[/itex]. With force and mass expressed in pounds-force and pounds-mass, one must use the more general form [itex]F=kma[/itex].

ps... if i weigh something on a scale do i consider that to be pounds-force or mass?
A balance scale require that the object to be "weighed" have some actual weight but measures the object's mass. Spring scales require require that the object to be "weighed" have some actual weight but measures the object's apparent weight.
 
  • #8
On some books I saw the equation F = m·a/gc, what does the gc stand for ?
 
  • #9
gc is fundamental constant that relates the pound force and pound mass. It allows one to use Newton's Second law with the units of mass as Lbm, not slugs.

[tex]g_c=32.1740 \frac{lb_m*ft}{lb_f*s^2}[/tex]
 
  • #10
GT1 said:
On some books I saw the equation F = m·a/gc, what does the gc stand for ?
It is the inverse of k in [itex]\mathbf F=km\mathbf a[/itex]. Newton's second law says that force is proportional to the product of mass and acceleration: [itex]\mathbf F \propto m\mathbf a[/itex], or [itex]\mathbf F=km\mathbf a[/itex]. SI units were designed so that the constant of proportionality is exactly one. The English pound force and pound mass (and pound sterling, for that matter) were defined a bit differently: The gravitational force exerted by the Earth on a mass of one pound (mass) at the surface of the Earth is one pound force (and in days of long ago, one pound mass of sterling silver was a pound (money)). Back to Newton's second law. An object dropped a short distance above the ground will accelerate at g=32.2 feet/second2. By Newton's second law, [itex]a = F/(mk)[/itex]. Thus [itex]k= 1/g[/tex], the acceleration due to gravity if one uses English units.
 

Pound Force to Kilograms Conversion

Converting between different units of measurement is a common task in various fields, including physics and engineering. Let's address some common questions about the conversion of pound force to kilograms:

Q1: What Is a Pound Force?

A pound force (lbf) is a unit of force commonly used in the United States and some other countries. It represents the force exerted by one pound of mass due to gravity. It is equivalent to approximately 4.44822 newtons (N) in the International System of Units (SI).

Q2: What Is a Kilogram?

A kilogram (kg) is the base unit of mass in the International System of Units (SI). It is equal to the mass of one liter of water at its maximum density. Kilograms are commonly used worldwide for measuring mass and are not to be confused with pounds (lb), which measure weight or force.

Q3: How Do I Convert Pound Force to Kilograms?

To convert pound force (lbf) to kilograms (kg), you can use the following conversion factor: \[1 \text{ pound force (lbf)} = 0.45359237 \text{ kilograms (kg)}\] Simply multiply the number of pound force units by this conversion factor to obtain the equivalent mass in kilograms.

Q4: What Is the Purpose of Converting Pound Force to Kilograms?

Converting pound force to kilograms is useful when you need to express a force value in a unit of mass. This conversion is commonly required in engineering, physics, and other scientific disciplines where mass and force are distinct but related concepts.

Q5: Can I Use Online Conversion Tools for This Conversion?

Yes, there are numerous online conversion tools and calculators available that can quickly perform conversions between different units, including pound force to kilograms. These tools can be convenient for quick conversions and calculations.

Q6: What Are Some Examples of Pound Force to Kilograms Conversions?

Here are a few examples of pound force to kilograms conversions:

  • 10 pound force (lbf) is approximately equal to 4.53592 kilograms (kg).
  • 25 pound force (lbf) is approximately equal to 11.3398 kilograms (kg).
  • 50 pound force (lbf) is approximately equal to 22.6796 kilograms (kg).
You can perform more conversions using the conversion factor mentioned in Q3.

In summary, converting pound force to kilograms is a straightforward process using a conversion factor. It is a common task in various fields and allows you to express force values in units of mass, which can be useful in scientific and engineering calculations.

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