The three laws of motion formulated by Sir Isaac Newton in the 17th century dictate the effect forces have on masses and how those masses move. The first law, the Law of Inertia, states that objects have a tendency to resist changes in their current state of motion. The Law of Acceleration states that the force of an object is equal to its mass multiplied by its acceleration, or that the amount of force needed to accelerate an object can be found by multiplying mass by acceleration. The final law, the Law of Reaction, states that every force applied results in an equal and opposite reactive force.
Gravity is one of the four fundamental physical forces that shape the universe, along with the strong and weak nuclear forces and electromagnetism. Like electromagnetism, gravity has infinite range. All objects exert a gravitational force proportionate to their mass; this pull diminishes with distance. Gravity, when paired with momentum, is responsible for objects' orbiting one another. The speed of an object's horizontal motion is enough to keep it in constant balance with gravity.
Two common measurements used when dealing with forces and motion are weight and mass. Weight is measured in "newtons," and represents the gravitational force exerted on an object: the mass of the object multiplied by gravitational acceleration. On Earth, this is 9.8 meters per second per second. Therefore, your weight varies depending where you are in the universe. On the moon, you would weigh about one-sixth of your weight on Earth because of weaker gravitational pull. Mass is a fundamental measurement, dictating how much matter an object has in it; its standard unit of measurement is the kilogram.
Velocity is the measurement of the speed and direction of an object's movement, and is usually expressed in meters per second. Acceleration is the rate at which velocity changes, expressed in meters per second per second. Any change in an object's velocity is acceleration. When someone says that a car "decelerates," he really means that it is accelerating negatively.