What do scales measure?

What do scales measure? Professor Mac is stepping on to grey scales with two close-up pictures shown to the left. One of the scales shows his weight in kilograms and one in Newtons.

Professor Mac steps on scales to measure his weight.

What do scales measure? The obvious answer is kilograms (or grams). That is what the reading on the scale indicates. However this is not what the scale is measuring. It actually measures force, and it is all to do with the way in which the scale is constructed.

Inside a mechanical scale is a spring. And springs are used to measure force. The way they measure force is through the fact that if you shorten the length of a spring, a force is required. The greater the amount you shorten the spring, the more the force that is required. So by measuring the change in length of the spring, you have an indication of the force applied.

In the scale, this change in spring length is used to move a dial which indicates the force from your weight acting on the spring. We measure force in the unit known as a newton. So your scale should display your weight in newtons. However it will normally indicate your weight in kilograms. But kilograms is actually a measure of your mass and this is where the confusion starts.

What do scales measure? Professor Mac's feet are on grey scales with his weight shown in newtons on a scale between his feet

Professor Mac stands on scales measuring his weight in newtons

 

What do scales measure? Professor Mac's feet are on grey scales with his weight shown in kilograms on a red scale between his feet.

Professor Mac stands on scales measuring his weight in kilograms

Mass and weight are not the same thing. You may think of mass as the amount of matter in an object. And objects with matter attract each other simply because they have mass. This is the result of the gravitational force of attraction between the masses of the objects. Our weight is simply the force on us due to the pull of the Earth on our mass. This pull force is directly related to our mass. So the larger our mass the larger the pull force from Earth. Since there is a direct relationship between mass and weight it is possible to convert the scale reading directly from newtons to kilograms and thereby indirectly measure our mass.

So how do you know that your scale is actually reading force and not mass? Well there is an easy way to confirm this. Stand on your scale close to a fixed object and then carefully push yourself down on the scale using the fixed object as a support. You will suddenly see the scale reading increasing, indicating that your mass has increased. However this is not the case since you haven’t suddenly put on mass. What you have done is added an extra force to the scale, and it is this force, combined with the force of your weight, that the scale has measured.

This simple experiment is an easy demonstration that a spring scale actually measures force and not mass.

Mac

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28 Responses

  1. kunal wadhwa says:

    Best way to understand ! Thanks a million Professor for your examples and quick responses to queries.

  2. horia says:

    Hi professor, don’t you think that the scale has been calibrated to measure mass instead of force.

    From your example one could infer that the scale calculates a person’s mass from the weight given to it.

    • The Professor says:

      Hi Horia. Yes the scale has been calibrated to display mass from an applied force on the scale. The internal mechanism inside the scale measures force. The printed label on the scale display has been calibrated to convert this into a reading of mass.

  3. chrisforts says:

    I have a mass, so do i produce gravity?
    Also, if i get rid of my mass, will i float?

    • The Professor says:

      Hi Chris. Yes you produce an attractive gravitational force between you and other objects since you have mass. If you have no mass you would have no force between you and Earth so in theory you would float if you were above the ground.

  4. Nethaji says:

    Thank you very much Professor. it was my very big confusion between weight and mass which is showing in scale.

  5. Manjunathan says:

    Dear Professor,
    Have a doubt on this subject, when I standing on a weight machine, it shows 80Kg.
    Now my daughter tried to pull me up, for her she can’t pull me up, but she is giving an upward with some force, I am still on the weight machine, does the weight machine will show lesser than 80Kg? or still, it will show 80Kg?

    • The Professor says:

      Hi Manjunathan. Assuming your daughter is not on the scales when she applies an upward force to you, she reduces the downward force on the scale, and therefore the scale reading will reduce (to less than 80kg). Since the scale is showing mass it will say that your mass is reducing. Your mass is not reducing but the force on the scale is reducing, and since it measures force you will see the scale reading reduce.

      • Manjunathan says:

        Thanks, Professor. I really appreciate your quick reply and making a correction in my understanding.

        If say my daughter is 20KG and standing on another separate scale, and when she tried to pull me up, where my scale reading is reduced by say 9.8N(1KG), in my scale it would be 79KG and in her scale, it would be 21 KG ? Basically overall MASS remains same?

        Thanks once Again.
        Manjunathan

  6. Leila says:

    But when I am weighing myself on a scale what am I measuring

    • The Professor says:

      Hi Leila. The scale is measuring the force between you and Earth. This is your weight. The reading on the scale may have been calibrated so that it converts this force into mass. Use the reading and units on the scale to determine what you have measured.

  7. sarath says:

    That was a very useful Information. Thank u very much.

  8. anderson says:

    Pounds are actually a measure of force — slugs being the related unit for mass. We can convert a scale “weight” of kg (mass) to pounds (force) by multiplying by ~2.2. So a kg scale is calibrated to give out its readings in mass but a pound scale gives out its readings in force? Don’t we have an apples and oranges situation here units-wise?

    • The Professor says:

      Hi Anderson, You can calibrate the reading of the scale in any appropriate unit. However the mechanism inside the scale is measuring force and that was the point that I was explaining.

  9. Sagar007 says:

    Then should I confidently say that my mass is 90 Kg weighed on weighing machine? & Weighing machine measures mass only not a weight?

    • The Professor says:

      Hi Sagar, yes if the weighing machine says your mass is 90 kg then that is your mass (assuming the weighing machine has been calibrated correctly). The confusion is that the weighing machine actually measures your weight, that is a force between you and Earth. We can calibrate the scale to show your mass since there is a direct relationship between your mass and your weight on Earth. See my video on gravitation here.

  10. Doug says:

    So if I weigh 98 kg on Earth is my mass 98 kg or 98/9.8 = 10 kg?

    • The Professor says:

      Hi Doug. Your weight is not measured in kg, that is your mass. On Earth if you have a mass of 98kg your weight is 98 x 9.81 Newtons. When you weigh yourself on scales the display is actually showing your mass in kg even though it is measuring your weight.

  11. Fred Fredericks says:

    If the scale reads in kg, how do you convert it to Newtons if the kg reading is not mass.

    • The Professor says:

      Hi Fred, you convert mass to force by multiplying by the acceleration of the object. On Earth you multiply by 9.81 m per second per second. The 9.81 represents the acceleration that the object has during free fall and results from the attractive force between the mass of the object and the mass of Earth. This attractive force is the weight of the object on Earth. See my video on Newton’s law of gravitation.

  12. Meriam says:

    So do we get a value that is divided by acceleration due to gravity or we get a value on the scale that includes the value of g

    • The Professor says:

      Hi Meriam, The scale is calibrated, which means that a force is applied to it and then the scale is created to provide this reading in kg to give a mass reading. You would need to divide the force by g to get the mass.

  13. Radhika Oka says:

    Very well explained. Thanks. Now I can use this explanation to help my students understand this concept

  14. Habeeb Ahamed says:

    I need to improve my physics knowledge by Mac website

  1. July 22, 2016

    […] http://www.learnwithmac.com/2015/03/27/what-do-scales-measure/ ..how do you know that your scale is actually reading force [weight] and not mass? Well there is an easy way to confirm this. Stand on your scale close to a fixed object and then carefully push yourself down on the scale using the fixed object as a support. You will suddenly see the scale reading increasing, indicating that your mass has increased. However this is not the case since you haven’t suddenly put on mass.   CWatters, Jul 22, 2016 at 2:32 PM […]

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