I'm reading 'Waves in an impossible sea' by Matt Strassler - a very good, popular science book, by the way - and he explains that the rest mass of a proton mostly comes from the kinetic energy of the quarks and gluons whizzing around. But I don't understand why there is quite so much energy in th...

In physics textbooks, problems about rolling usually include the assumption that the rolling is without slipping, meaning that $v=R\omega$ throughout the motion, where $v$ is the translational velocity, $\omega$ is the angular velocity and $R$ is the radius. I do not understand the justification ...

Say I have a particle travelling in the $x$-direction and breaking into two pieces: Conservation of momentum in the $x$-direction obviously gives: $$mv = m_1 v_1 \cos(30) + m_2 v_2 \cos(60).\tag{1}$$ In the $y$-direction, I could write the following: $$0 = m_1 v_1 \sin(30) + m_2 v_2 \sin(60).\ta...