According to Sir Issac Newton's first law of motion, an object at rest stays at rest and an object in motion will stay in motion in a straight line and at a constant speed, unless an external unbalanced force is applied. This is known as uniform motion. A force is any influence on an object that causes a change in its speed, direction or shape. Force is also a vector quantity, having both magnitude (amount of force) and direction. Types of forces include friction, gravity and applied force.
A change in velocity is caused simply by applying an external unbalanced force. For example, if you are driving a car and are traveling at a constant speed, you can change the car's velocity by steering the vehicle, thereby causing a change in the direction of friction that is exerted on the tires. This causes the direction in which you are traveling to change and thus causes a velocity change. Similarly, a soccer ball undergoes a velocity change when it is kicked.
Though most often used to describe an increase in speed, the term "acceleration" actually applies to a change in velocity, be it an increase or decrease in speed or a change in direction. For example, an object that is traveling in a circle at a constant speed is actually undergoing constant acceleration, since it's continually changing direction. Likewise, a ball rolling to a stop is accelerating, as its speed is decreasing. In physics and engineering fields, a changed in velocity is known as a "delta-V."
Velocity is expressed in either feet per second (fps) or meters per second, depending on whether the customary or metric system is used. Changes in velocity are expressed as feet or meters per second per second, or fps squared. This describes the rate at which an object's velocity changes. For example, on Earth, gravity causes falling objects to accelerate at a rate of 32.2 feet per second per second, meaning that if an object is dropped, after the first second it will be falling at a rate of 32.2 feet per second. After two seconds, it will be falling at a rate of 64.4 feet per second.