Force is defined as an interaction that causes an object to change its motion, either by speeding up, slowing down, or changing direction. Force is a vector quantity that has both magnitude and direction, with the SI unit being the newton. Newton's three laws of motion state that an object at rest stays at rest unless acted on by an unbalanced force, acceleration is directly proportional to net force and inversely proportional to mass, and for every action there is an equal and opposite reaction between interacting objects.