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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10 Responses

  1. 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.

  2. 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.

  3. 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.

  4. Radhika Oka says:

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

  5. 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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