As the dragster in the picture is leaving behind a cloud of smoke at the start line, it is "accelerating", in the everyday sense of the word, meaning its speed is increasing.

In physics, words that have a certain meaning in everyday life often acquire a more specific and often somewhat different meaning. The word "acceleration" is one example. In physics, an object has acceleration when its speed is increasing or when its speed is decreasing. An object can even have acceleration when its speed stays the same!

What then is acceleration in physics? - In physics, one says that an object is accelerating when it changes its state of motion, in other words, when it changes its velocity. In physics, acceleration is defined as the time-rate-of-change of velocity.

Thus, an object is accelerating either when its speed is changing (increasing or decreasing - yes, even if the speed is decreasing does one say in physics that the object has acceleration!) or when its direction of motion is changing. Remember that velocity is a vector quantitity. Therefore we must consider changes in either its magnitude or direction.

The dragster in the picture is accelerating because its speed is changing - the speed is increasing at this moment. The direction of the dragster's velocity is not changing - hopefully for the driver.

On Page 2, you can observe the acceleration of an object whose velocity is changing direction, namely, the acceleration of a planet. You may want to go to the Link under Related Items and carry out the "Activity on Acceleration" with the applet. Instructions on how to use the applet are available under Applet Help on the applet's Help menu. See the files called Help and ShowMe.