Force is a vector; it has both direction and magnitude. The SI unit for force is the newton (N). One newton of force is equal to 1 kg * m/s2, where the “*” symbol stands for “times.” Force is also represented by the symbol F. Newton’s First Law of Motion says that an object will continue to move at a constant velocity unless it is acted upon by an external force. Objects in motion remain in motion until a force acts on them. This is inertia. They won’t speed up, slow down, or change direction until something acts on them. Force is proportional to acceleration, which is defined as the rate of change of velocity. In calculus terms, force is the derivative of momentum with respect to time. The concept of force was originally defined by Sir Isaac Newton in his three laws of motion. He explained gravity as an attractive force between bodies that possessed mass. However, gravity within Einstein’s general relativity doesn’t require force. Newton’s Second Law of Motion says that force is directly proportional to acceleration (the rate of change of momentum) for a constant mass. Meanwhile, acceleration is inversely proportional to mass. This law is useful for measuring forces. If you know two of the factors, you can calculate the third. You also know that if an object is accelerating, there must be a force acting on it. Newton’s Third Law of Motion relates to interactions between two objects. It says that for every action there is an equal and opposite reaction. When a force is applied to one object, it has the same effect on the object that produced the force but in the opposite direction. The action and reaction forces happen at the same time. If you pull on a rope, the rope is pulling back on you.