# Pounds per square inch

If I have something that is can take 130 pounds per square inch does that mean that it takes that much weight it can hold before it breaks? So if something is spread over 4 inches does that mean it can hold up 520 pounds?

In: Physics

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Pounds per square inch is a quite rare measure of pressure. What you described is basically correct, but not the only application.

PSI

Pounds Per Square Inch

Pounds / Square Inch

It’s a measurement of pressure. A Force divided by an Area.

If you weigh 200 pounds, and the bottom of your shoe has an area of 20 square inches, then standing on your two feet would exert 200 pounds / (20 square inches * 2) = 5 pounds/square inch.

If you are talking about a floor, think of a floor made of thin ice, of glass, of tile, and of wood. Each of these can withstand different pressures. Standing on the thin ice would easily break it, the glass could crack, but the tile and wood could be fine.

You can also think about changing the size. Lets say I switch the two shoes to two peg legs. The end of each peg leg is only one inch square. Now each peg leg is exerting 100 pounds per square inch.

Or I get the point even sharper so it’s 0.1 square inch. Now I’m exerting 1000 pounds per square inch on the floor.

If I’m holding a pencil and pushing the eraser into the palm of my hand with 10 pounds of force, that’s not going to do much of anything. But if I take a needle and push it with 10 pounds of force it will go right into my skin because the pressure of 10 pounds on a tiny area is enough to break my skin.

Not sure of the context here but be aware that if you’re speaking in terms of floor load (on a deck or the 2nd floor of a house, for instance) the PSI rating doesn’t apply to the entire surface area.

That PSI rating has more to do with puncturing the decking material than the total weight the floor can bear, which is a different figure, called a load rating.

For instance, a 10’x10′ room in a house with a 110PSI rating can not support 1,584,000lbs in total. (120″x120″x110psi) That’s a different rating determined by the joist dimensions/placement and decking method.

Neither can it support a 500lb shelving unit with 1/2″ square feet. (0.5″ x 4 feet x 110PSI = 220lbs)

*edit: a word

Have you ever had a cat fall asleep on your chest? It’s cozy, because you have 20 lbs of cat spread out over the entire surface of the cat – let’s just say 20 sq inches, so that’s 1 lb/sqin

However, when that cat wakes up, stands up and stretches, you will think your chest is going to implode. Now 20 lbs of cat is being supported by 4 tiny paws – let’s say each paw (cats stand on their tippy toes) is about 1/2 in sq. So that is 20 lbs of cat now focused on just 2 sq in of surface area. 10 lb/sqin

Take a 1 inch cube and put a 1 pound weight on top of it. The force on the bottom of the cube is now 1 pound per square inch.

At least that is how is was explained to me.

It’s important to note that PSI is a unit of measurement for *pressure.*

You take a *force* (measured in pounds) spread out over an *area* (measured in square inches) to caluculate the *pressure* (measured in PSI).

Pressure is equal to force divided by area

Force is equal to pressure times area

Area is equal to force divided by pressure

If you know any two of these three values, you can calculate the third.

Imagine two scenarios-

In both scenarios you weigh 100lbs.

in scenario 1, your feet are 1” long and 1” wide so each foot has a surface area of 1” squared (I don’t know how to do the power of 2 symbol on my phone, sorry)

in scenario 2, your feet are 5” long and 2” wide so they have a surface area of 10 square inches

You want to walk across a frozen lake. The ice is thin and your scientist buddy says the ice can handle a maximum pressure of 60psi.

In scenario 1, your 100lb body weight (force) is acting on a 1” area to create a pressure of 100 pounds per square inch. You’d break through the ice.

In scenario 2, your 100lb body weight (force) is acting on a 10” area to create a pressure of 10psi. You’d be able to walk across the ice without breaking through.

So you were right in your question. If something can handle 130psi, it can handle more than 130lbs if you spread it across a larger area than 1”. PSI=pounds per square inch.

Imagine trying to push your finger through your hand, it’d be pretty hard. But if you try to push a needle through it’d be easier because the same force is applied over a much smaller area

Yes, more or less. That is how pressure works. The amount of force spread over a surface of contact. It’s why using the same amount of force will distribute force and sting less than if you did the same thing, but with a finger.. or a balled up fist.