# A Newton spring balance

Professor Mac is using a Newton spring balance in his experiments to illustrate Newton’s second law of motion.

Do you know what it is?  Well it is shown below. The balance contains a spring which provides a measure of the force applied to the balance.  The larger the force, the more the spring extends, with the force level indicated by the scale on the balance.

## The Newton Spring Balance and Hooke’s Law

The Newton spring balance uses a law known as Hooke’s Law to relate the spring extension to the force being measured. There is a direct relationship between how much the spring extends and the force required to extend it.

If we place a weight at the end of the spring there is a force applied to the spring due to the weight. Hooke’s law says that the spring will then extend. As it extends you can measure the force due to the weight by looking at the scale. In the picture below you can see Professor Mac applying a force to the end of the spring using his hand. The spring extension and force applied is indicated on the scale.

## Before you go!

So do you know how to determine how much a spring extends when a force is applied to it? We need to know this if we are going to use it to measure the force.

This is done through a process of applying various forces to it and measuring the amount it extends for each unit of force. This enables us to calibrate it and create a scale which indicates force. Professor Mac shows you how to do this in his video on Hooke’s Law. Mac

I teach physics and engineering through animation.

### 55 Responses

1. Kgau Molele says:

On a free body diagram do we represent the spring balance as tension force or fg(weight)?

• Mac says:

If you are doing the free body diagram of the object at the end of the spring balance then the force on the object from the spring balance can be represented by a “force spring on object”. You can then solve for this force.

2. LIEZL says:

what was the effect on the spring balance as you add more padlocks?

• Mac says:

Sorry Liezl I don’t understand your question. Can you be more specific what you are asking?

3. Alannah says:

how do you read a newton scale with different ascending values?

• Mac says:

Sorry Alannah, I don’t understand your question. Can you be more specific?

4. Lahni says:

Hi Professor,
Trying to understand the risk assessment of this experiment. What would an example be? Thank you

• Mac says:

Hi Sammi. Think about the weights. What might happen?

5. Mohisn says:

How could you alter the scale on the spring balance so that you could read the correct mass from it directly?

• Mac says:

Hi Mohisn. Think of the relationship between mass and weight on Earth. There is a constant relationship. Once you have the constant you can use that to change the scale. I won’t tell you the answer directly so please use these clues to work it out.

6. whiteturdle21 says:

how could you alter the scale on a spring balance so that you could read the correct mass directly?

• Mac says:

You would calibrate it with known masses. Use the same process as shown in my video with mass rather than force.

7. Kenya says:

So… It Might Depend Of The Weight Or it Depends On The Item?

• Mac says:

It only depends on the weight of the item not what it is.

8. Sithulisiwe says:

Professor if we add another spring balance to two spring balances what will be the reading on each spring balance using the magnitude of 5N if spring balance A is 5N

• The Professor says:

Hi Sithulisiwe. I assume you mean use two spring balances to hold up the mass with weight 5N. If you use two spring balances in parallel (that is they are not joined end to end) then each spring balance would carry half of the weight. So each would read 2.5N.

9. grace22 says:

what was the effect of the spring balance as you add more pudlucks?

• The Professor says:

Sorry Grace I don’t understand your question. What do you mean the effect of the spring balance?

10. Roxxi says:

Very good animator! Amazed, how do you do it?

• The Professor says:

Hi Roxxi. I use the Blender software. See here for details: http://www.blender.org

11. maydelle says:

what is meant by “zero” reading in the spring balance?

• The Professor says:

Hi Delle, a zero means there is zero force on the end of the spring balance.

12. Yokai says:

Hi!what do the values represents on 2 spring balances?

• The Professor says:

Hi Yokai, the values on the scale of a spring balance can represent several measurements. Normally it will be a measure of force. The unit of force on the scale will depend on the calibration of the spring balance. You can calibrate the scale to measure newtons and this is what is shown on the two sides of the scale in the picture above. See my post here to understand how to calibrate a spring balance. Mac

13. Tron Breezy says:

hi, i did an experiment which is to pull an object by using a newton scale..And the force used to pull the object is less than the friction force acting against the object..Can you elaborate on what is happening in the experiment?

• The Professor says:

Hi Tron, You need to look at the forces on the object you are pulling. Add them up and you will see why the force applied by the spring balance is less than the friction force. Mac

14. Jaiden says:

In newtons does it have .4 or .9 in between numbers

• The Professor says:

Hi Jaiden, Yes you can have decimal places for the measurement of Newtons. The scale shown does not have these to make it easier to read the scale in the videos. Mac

15. Bigom Martin says:

Literature review on the use of spring balance in teaching the concept of weight

16. tulsi says:

what are the use of spring balance

• The Professor says:

Hi Tulsi, the spring balance is used to measure the weight of objects. The key point to note is that weight is a force and spring balances are used to measure force. What is not so obvious is that the spring balance doesn’t measure force directly, instead it changes length as a result of force being applied to it. By relating this change in length to the force applied we can create a scale which enables us to measure force.

Most weighing scales use a version of the spring balance to measure weight.

Mac

17. natalie says:

im writing back ground information and I don’t know how to explain what a newton exactly is

• The Professor says:

Hi Natalie, A Newton is a unit of force. From Newton’s second law of motion F=ma, 1 Newton will accelerate a 1kg mass at 1m/s/s. You can’t see a force, you can only see the effect of a force, which will change the motion of an object if it is non-zero. Mac

18. vikash tiwary says:

Two persons exert equal force of 20 kg-wt from bith ends of the spring balance.what is the reading of the spring balance.

• The Professor says:

Hi Vikash, Draw a fee body diagram with the forces on the spring balance and see if you can work out the answer. Mac

19. Sir-Abu says:

What is the weight of metal block? If a metal is hanged in air and the reading is 9.6N on a spring balance.

• The Professor says:

Hi Sir-Abu, The weight is 9.6N as weight is a force. If you are asking what the mass is then you need to calculate this. Refer to your notes from your teacher to see how to do this. Mac

20. simonia says:

what are the sizes of force on a spring balance?

• The Professor says:

Hi bsa, I don’t understand your question. The size of the force depends on the size of the force applied to the spring balance. The unit of measurement of force is the newton.

Mac

21. Wennia WhIta says:

I have 2 questions.
1）why a newton balance can measure the force of gravity？It can only measure the force we exerted ，isn’t it？can you explain the ans ？
2）what is the difference between weight and force of gravity？some said that they are the same.But if they are the same，why there is no “F=Weight’？
Thanks a lot！

• The Professor says:

Hi Wennia, thanks for your questions.

In your first question you recognise that a newton balance measures force. However the important point to recognise is that the newton balance does not distinguish between how the force is applied. It responds to an applied force if you pull it or if an object (with mass) is hung from the balance. When a mass is hung from it the weight of the mass applies a force to the newton balance. Note that weight is a force and it is created by the gravitational attraction between the mass and Earth.

Question 2: The weight of an object is created by the gravitational attraction between it and Earth. So there is no difference between the weight of an object and the force of gravity on it – it is the same thing. We do use the statement force = weight. Think about when you step on scales to measure your weight. The force on the scales is your weight. I have written a blog post about it here.

Regards Mac

22. Rohit says:

Hi, If we have two same spring balances in series and a single weight of 100 N hanging in the end then would the spring balances would measure 50 N each ?

• The Professor says:

Hi Rohit, First think about what a spring balance measures. It measures the force applied at the end of the spring balance. If you consider the weight hanging from one of the spring balances it would read a force of 100N. Assume the spring balance has zero weight. If you now hang the first spring balance (still holding the weight) from the end of the second spring balance, what weight is hanging from the second spring balance? Just think of weight. That will give you the answer.

23. Frannie says:

I know that force is measured in newtons but how did they manage to express that on a scale? In other words how much is that ‘unit length’ or how did they come about to calculate that unit length?

• The Professor says:

Good question Frannie. If you hang a 1kg mass from the spring balance and measure the extension you can calibrate the scale. That is you know that a 1kg mass exerts a force of approximately 9.81 Newtons due to its weight. So you then know the extension for 9.81 Newtons and therefore you can calculate the extension for 1 Newton. You do that by dividing the measured extension when the 1kg mass was applied by 9.81 Newtons which gives mm per Newton (assuming you measured the extension in mm).

Regards, Mac

24. Alex says:

Hi! I have a science fair soon and I need a newton spring balance for my project
Do you know how much 1 costs?
Alex

• The Professor says:

Hi Alex, Just search the web for images on “Newton spring balance” and then you will find some sites selling them. They are around 4 British Pound Sterling (6.5 USD).

Make sure you get the right capacity of balance for the loads you are going to measure. Good luck with your project.

Mac

25. emma says:

hi i am a science test 2morrow and i need to kinow a bit about a newton spring balance if you could leave me some information please that will be great thanxs:)

• The Professor says:

Hi Emma, my video on the spring balance has unfortunately been put on hold while I develop my video for Newton’s third law.

The Newton spring balance measures force through applying the principle of a law known as Hooke’s law. Simply stated, Hooke’s law describes that the force applied when deforming something is proportional to the size of the deformation. So if you pull on a spring you apply a force. The spring extends by an amount (note that the amount is different for every spring). Now if you double the applied force the displacement of the spring will double. In other words it is a linear response. We can use this fact to calibrate the spring which means that we determine how much force is required for a unit length change of the spring. Knowing this we can measure the length of the spring for an applied force and then calculate the force applied by using the knowledge of how much force is required per unit length spring extension. That is how a Newton spring balance works.

Regards Mac

26. Ellie says:

What is it exactly??

• The Professor says:

Hi Ellie, The Newton spring balance is used to measure force. Have a look at my video on Newton’s second law of motion and you will see it in action. Mac

27. intan says:

how to know const. spring from this dimention

• The Professor says:

Hi intan,

To calculate the spring constant you need to know two values; a force level applied to the spring and the extension that is produced by that applied force. By dividing the applied force by the spring extension you get the spring constant. Each spring has a different spring constant so you need to do the calculation for each spring. In the picture above there is no information about the applied force so you cannot calculate the spring constant for it.

I am producing a video which explains this calibration process in detail but it will be a few months until it is released. Mac

28. Preksha says:

please show this experiment with full steps like aim, materials, procedure, observation, and result……….

• The Professor says:

Hi Preksha,

I will be starting the production of a video on Hooke’s law very shortly. This will use the spring balance to demonstrate the relationship between force and extension of the spring. If you watch my current video on Newton’s Second Law you will see the spring balance used to measure force.