# Acceleration

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Acceleration is the rate of change of velocity, meaning that if an objects speed or direction is changing, it is undergoing an acceleration.[1] The SI unit for acceleration is m2/s. Acceleration is felt by an object when the net force acting on it is non-zero.[1] 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:[1]

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, the motion feels constant when standing or sitting still for the most part—until the vehicle's speed or direction changes.[1] 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 a common experience on Earth, as objects falling towards the Earth pick up speed as they fall (however, they are limited by air drag).[1]

## PhET Simulation on Forces

The University of Colorado has graciously allowed us to use the following PhET simulation. Explore out the PhET animation below to explore how forces cause acceleration.