Question

Why is the sum of masses of quarks not equal to mass of proton?

Answer 1

Answer:

When we talk about the mass of quarks and protons, we mean their mass when they aren't moving. This is called the "rest mass." As you noticed, the rest masses of two up quarks and a down quark don't add up to the rest mass of a proton. They only account for about 1% of the proton's mass.