Newton's second law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. It can be expressed by the equation F=ma, where F is the net force, m is the mass of the object, and a is the acceleration. The document provides an example problem of calculating the force needed to push a car with a mass of 1000 kg and an acceleration of 0.05 m/sec^2. Using the equation, the force is calculated to be 50 N.