A Scale Doesn’t Always Tell You the Mass of an Object

Photo: Rhett Allain

Let’s start with a two part physics question. Suppose I have an inflated basketball and I place it on scale to measure its mass (call this ball A). Next, I let the air out of the ball and put it back on the scale (ball B). Finally, I squish the ball down so that it’s no longer spherical and put it on the scale (ball C).

Question: Which scale will read the highest value and which will read the lowest?

Question: Which ball will have the greatest weight and which will have the lowest?

Now for some physics.

What Does a Scale Measure?

Yes, we have the scale setting on “grams” — but does it really measure the mass? Nope. It doesn’t.

Suppose we have a scale that’s just a spring. When we place an object on the “scale” it compresses the spring. The more the spring compresses, the greater the “reading”. But what makes the spring compress? It’s not the mass, it’s the gravitational force. Here’s a force diagram to show you how this would work.

Newton’s second law says that the net force is equal to the product of the mass and…