According to Newton's second law of motion, the force acting on a body is directly proportional to the rate of change of its momentum. Mathematically, this is expressed as:

\([F = \frac{dp}{dt}]\)

where F is the force, p is the momentum, and t is time. When the linear momentum (p) is constant, its rate of change (dp/dt) with respect to time is zero. Therefore, the net force acting on the body must also be zero.