Newton's second law states that acceleration is directly proportional to force and inversely proportional to mass. It also specifies that an object accelerates in the direction of the applied force. Force is calculated using the formula F=ma, where F is force in newtons, m is mass in kilograms, and a is acceleration in meters per second squared. Centripetal force is any force directed toward the center of an object's circular path, such as the force exerted by a skater's partner to keep them moving in a circle.