If the signs of the velocity and acceleration of a particle are the same, the speed of the particle increases. If the signs are opposite, the speed decreases.
5 An Acceleration's Sign In common language, the sign of an acceleration has a nonscientific meaning: positive acceleration means that the speed of an object is increasing, and negative acceleration means that the speed is decreasing (the object is decelerating). In this book, however, the sign of an acceleration indicates a direction, not whether an object's speed is increasing or decreasing.
For example, if a car with an initial velocity v = -25 m/s is braked to a stop in 5.0 s, then aavg = +5.0 m/s2. The acceleration is positive, but the car's speed has decreased. The reason is the difference in signs: the direction of the acceleration is opposite that of the velocity.
Here then is the proper way to interpret the signs:
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If the signs of the velocity and acceleration of a particle are the same, the speed of the particle increases. If the signs are opposite, the speed decreases. |
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