Читать книгу Physics I For Dummies - Steven Holzner - Страница 63

Just as you can examine average and instantaneous speeds and velocities, you can also examine average and instantaneous acceleration. Average acceleration is the ratio of the change in velocity to the change in time. You calculate average acceleration, also written as , by taking the final velocity, subtracting the initial velocity, and dividing the result by the total time (final time minus the initial time):

At any given point, the acceleration you measure is the instantaneous acceleration, and that number can be different from the average acceleration. For example, when you first see red flashing police lights behind you, you may jam on the brakes, which gives you a big acceleration, in the direction opposite to which you’re moving (in everyday parlance, you’d say you just decelerated, but that term’s a no-no in physics circles). But you ease up on the gas pedal a little and coast to a stop, so the acceleration is smaller. The average acceleration, however, is a single value, derived by dividing the overall change in velocity by the overall time.

Acceleration is the rate of change of velocity, not speed. If a velocity’s direction changes without a change in speed, this is also a kind of acceleration.