That statement is true.
Prof Newton said: Force = (mass) x (acceleration)
-- If the force is constant, then the right side
of the equation must be constant too.
-- The right side of the equation is the product
(mass) x (acceleration) .
-- If either of those changes, then in order to keep the product
constant, the other one has to change in the opposite direction.
-- So if the mass increases, then the acceleration decreases.
