# Why are gravitational mass equal to the mass in Newton's second law [duplicate]

Newtons second involves the mass $$m$$ e.g. $$F = m a$$

And the gravitation force also involves the mass e.g. $$F = - G \frac{m_1 m_2}{r^2}$$

The question is why the gravitation mass is equal to the mass in Newton's second law.