What is a spring constant of 20 N/m used for?

[DarkEnergy890]

Homework Statement

What is a spring constant of 20 N/m used for?

Relevant Equations

F=-ks

In my physics lab we determined the spring constant of a steel spring. This turned out to be 20 N/m. However, when I search online, I can't see any uses of springs - I know springs can be used everywhere, but nobody seems to specify their spring constant. Anyone know of any applications?

 

[kuruman]

A spring constant is a measure of how stiff a spring is. Different springs have different spring constants which could be determined by hanging various masses and seeing by how much the spring stretches. You made a measurement and, for the spring you were given, you determined its spring constant. Knowing the spring constant, you can predict how much force the spring will exert if it is stretched (or compressed) by a given distance. In you case, if the spring is stretched by 10 cm, it will exert a force of 2 N.

Note that doorbells are also used everywhere but nobody seems to specify online how loud they are either. You made a measurement no different from determining how loud a doorbell is by using a decibel meter.

 

[DarkEnergy890]

A spring constant is a measure of how stiff a spring is. Different springs have different spring constants which could be determined by hanging various masses and seeing by how much the spring stretches. You made a measurement and, for the spring you were given, you determined its spring constant. Knowing the spring constant, you can predict how much force the spring will exert if it is stretched (or compressed) by a given distance. In you case, if the spring is stretched by 10 cm, it will exert a force of 2 N.

Note that doorbells are also used everywhere but nobody seems to specify online how loud they are either. You made a measurement no different from determining how loud a doorbell is by using a decibel meter.

ya, but are there any applications with around that spring constant? With your doorbell analogy, yes, that is strange. I would sure like to know how loud my doorbell will be!

 

[berkeman]

ya, but are there any applications with around that spring constant?

 

[berkeman]

But car suspension is an application, which is what you were asking for, no?

 

[DarkEnergy890]

But car suspension is an application, which is what you were asking for, no?

Sorry, I didn't watch the video but saw it now. However, their value is around 50 N/m whereas mine was 20N/m. Could a suspension be used with a 20N/m spring constant?

 

[berkeman]

ould a suspension be used with a 20N/m spring constant?

For a lighter vehicle, sure. You need to use the spring constant in your calculations for the weight of the application, and how far you want the spring to compress or expand. You want the travel to be well less than the spring's __________ limit. (fill in the blank please)

 

[DarkEnergy890]

For a lighter vehicle, sure. You need to use the spring constant in your calculations for the weight of the application, and how far you want the spring to compress or expand. You want the travel to be well less than the spring's __________ limit. (fill in the blank please)

thanks - elastic

 

[nasu]

I know springs can be used everywhere, but nobody seems to specify their spring constant. Anyone know of any applications?

Of course they do. If you look up to buy springs you will see the parameters, including the spring constant.
https://www.mcmaster.com/products/springs/compression-springs-7/
Here is an example. They use lb/inch instead of N/m but it is the same physical quantity even though they call it "spring rate".

 

[DarkEnergy890]

Of course they do. If you look up to buy springs you will see the parameters, including the spring constant.
https://www.mcmaster.com/products/springs/compression-springs-7/
Here is an example. They use lb/inch instead of N/m but it is the same physical quantity even though they call it "spring rate".

but I meant for applications, e.g. in a pen, they don't give the spring constant. with that website you linked, it doesn't give applications, only spring constant.

 

[Chestermiller]

Do you have a ballpoint pen? How do you think they chose the spring for the pen? Do you think they chose it so that the force to click the pen was 100 N?

 

[DarkEnergy890]

Do you have a ballpoint pen? How do you think they chose the spring for the pen? Do you think they chose it so that the force to click the pen was 100 N?

Hmm, I can think of a way to measure it by hanging masses off the spring - problem is the spring is too small to hang stuff from. I don't have an accurate way to apply a force. Also, I would probably need to measure it with a large displacement to get an accurate value. It probably takes around 0.5kg to compress it 1cm? Therefore spring constant is probably like 500N/m, which is way different from my 20N/m value i calculated.

 

[renormalize]

but I meant for applications, e.g. in a pen, they don't give the spring constant. with that website you linked, it doesn't give applications, only spring constant.

Well, according to this table from chapter 9.5 of Essential Physics (https://www.amazon.com/dp/1937827038/?tag=pfamazon01-20), a spring constant $k$ of $20 \text{ N/m}$ is roughly that of a small rubber band. So your steel spring is only useable with very light loads.

 

[nasu]

but I meant for applications, e.g. in a pen, they don't give the spring constant. with that website you linked, it doesn't give applications, only spring constant.

They don't give it but the pen designer knew it. Why would you expect it to be given with the pen? Do you expect to get also the density of the material of the pen body, the viscosity of the ink, the molecular weight of the plastic used for the push button, etc?

It is the designer's job to know what spring constant is needed for his application. The website blongs to a hardware seller. They offer what they have and you pick whatever fits your needs.

 

[kuruman]

Also, I would probably need to measure it with a large displacement to get an accurate value.

How accurate a value do you need and why? Would knowing the value less accurately affect what you want to do with it? These are design questions that only you, as the designer, can answer.

 

[Chestermiller]

Hmm, I can think of a way to measure it by hanging masses off the spring - problem is the spring is too small to hang stuff from. I don't have an accurate way to apply a force. Also, I would probably need to measure it with a large displacement to get an accurate value. It probably takes around 0.5kg to compress it 1cm? Therefore spring constant is probably like 500N/m, which is way different from my 20N/m value i calculated.

A spring of a desired spring constant can be designed using elastic structural analysis.

 

[Mister T]

Hmm, I can think of a way to measure it by hanging masses off the spring - problem is the spring is too small to hang stuff from.

No it's not. Just hang small stuff from it. Or you can compress the spring instead of stretching it.

 

[Steve4Physics]

Might be worth researching 'microswitches' - which use small springs.

Also, it's worth noting that a small spring-constant does not necessarily mean a small spring. A 'slinky' is pretty large and will have a small spring-constant.

 

[Bruzote]

I think what you need to work on first is a basic business problem of trying to sell a complex product. Once you find you have to pay for everything used in production, you will want to know the best capabilities for every item you buy including springs. You would want to know what kind of spring is the cheapest yet accomplishes the most with its design. That requires knowing different spring geometries, materials that give you enough but not too much force, and what limits of maximal movement and endurance (time, repetitions, even corrosion) are needed.