In physics, a force is defined as an interaction that can cause an object to accelerate, change its velocity, or alter its trajectory. Force is a vector quantity, having both magnitude and direction, and is measured in newtons (N) in the International System of Units (SI). According to Newton's second law, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Forces can result from various interactions, including gravitational, electromagnetic, and contact forces.