What do scales measure?
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.
A mechanical scale
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.
Mass and weight
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.
What do scales measure?
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.
Before you go!
Can you explain what gravity is? You’ll be surprised how easy it is to explain if you take a look at my video on Newton’s Law of Universal Gravitation.