Acceleration
Acceleration is the rate of change of velocity, meaning that if an objects speed or direction is changing, it is undergoing an acceleration. The SI unit for acceleration is m2/s. Acceleration is felt by an object when the net force acting on it is non-zero. An object is said to be in constant motion when its velocity isn't changing, therefore the acceleration must be zero. Acceleration is represented by the equation:
where:
is the change in velocity
is the change in time
is the acceleration
The arrow above velocity and acceleration indicates that it is a vector, meaning it has a direction associated with it.
Acceleration in common experience
Acceleration is felt frequently in our daily lives. When riding in a moving vehicle, it feels as if you are standing or sitting still for the most part, until the vehicle's speed or direction changes. As soon as this happens, the force caused by this acceleration is felt by your body, as it is pushed to keep up with the changing velocity of the vehicle. In order for your body to accelerate with the vehicle, a seat is required to provide the push on your body. This is why people standing on a train or bus are susceptible to falling, because when the vehicle accelerates there is no object providing the force needed to match the acceleration. This is the same reason why tossing an object in the air in an accelerating vehicle isn't a good idea.
All objects with mass exert a gravitational force on other massive objects, which means that there is an acceleration due to gravity between any two objects. This is of common experience on Earth, as objects falling towards the Earth pick up speed as they fall (limited by air drag, however).