Newton's second law shows that there is a direct relationship between force and acceleration. It states that "Force acting on a body is equal to the product of its mass and acceleration"
or, F = m a
We know that force acting on a body is equal to change in moementum per unit time (from Newton's second law)
Thus, when the momentum of a body remains constant force acting on it will be zero.
F = 0 , when momentum remains constant.