Ultimately, all motion is defined in terms of distance and time. In one-dimensional motion speed is the distance traveled in a given time -- hence, the units "miles per hour," "feet per second" or, in metric, "kilometers per hour" or "meters per second." Acceleration, the fourth fundamental quantity in kinematics, is defined as the rate at which speed (or velocity) changes. From rest a body accelerating at five feet per second per second will be traveling at 50 feet per second after ten seconds and 300 feet per second after one minute.
The words "speed" and "velocity" are often used interchangeably by people using one term but meaning the other. Put simply, velocity both defines the rate at which distance is covered by a body and the direction in which this body is traveling. Speed is simply the rate of distance covered.
Velocity is called a vector quantity while speed is called a scalar quantity. Acceleration can also be a vector quantity, and when a body starts to slow down, it undergoes negative acceleration, which is also called deceleration or retardation.
The speed, velocity and acceleration of a body can be explained by the forces that act upon it. The analysis of these forces is called dynamics, and the movement of a body due to forces is defined by Issac Newton's Laws of Motion.
Newton's three laws of motion are remarkably simple and can be seen in action all around us. Newton's First Law states any body will be at rest or move with constant velocity when all forces acting upon it are equal. A car remains at rest because there is no force propelling it forward. A car moves at constant velocity because the force moving it forward, due to the engine, is equal to the resistive forces acting against it, which include air resistance, friction with the road and, if traveling up an incline, gravity. When a car travels down an incline, the force of gravity acts with the force of the engine -- which is why it takes more effort to travel up rather than down.
The Second Law defines acceleration in terms of force. When a force acts on a body, it will accelerate. The rate of this acceleration of a body is proportional to the force acting upon it and inversely proportional to its mass. A light body will accelerate more than a heavy body when the same force acts upon it. This is why a full car accelerates more slowly than a car containing just the driver.
The Third Law states that every action has an equal and opposite reaction. You are standing or sitting in front of your computer reading this article because the force of seat or floor against your body is equal to the force due to gravity that is pulling you downward. If these forces were not in equilibrium, you would either fall through Earth or accelerate off into space.
All bodies in motion can be described and their behavior predicted by Newton's laws. The calculations for placing satellites in orbit, predicting when the sun will rise or sending men to the moon all derive from the same three laws.