Originally Posted by Murexway
Quote:
You're talking about personal anecdotal evidence. I suspect people who believe this mistake a significant lack of deceleration as acceleration
Nope. Acceleration is acceleration. What's being perceived, of course, is the decrease in the rate of deceleration, but another way of expressing a negative negative is positive. A decrease in deceleration is......acceleration.
Well, technically you are correct (i.e., acceleration IS acceleration) - but not
exactly the way you've described it. Acceleration is velocity vs. time. As long as your velocity (defined as speed AND direction) is changing, you are "accelerating;" that is why a constant speed while changing direction is also called "acceleration." Typically, the term "acceleration" simply means a change in the rate of speed with respect to time. The term "deceleration" is a term of convenience that is used instead of saying "accelerating in a negative direction." When you are slowing down (whether or not you are maintaining a given direction) your speed is changing with respect to time. The term used to define this circumstance (a change in speed with respect to time) is, indeed, "acceleration." However, this is not describing a "negative-negative." What is happening is simply a reduction in the velocity with respect to time. When the slope of a speed vs. time chart is negative (down), you are progressively changing the distance you cover in a given amount of time ... so that you cover
less distance in each successive time interval - decelerating. When the slope of the line is positive (up) you are progressively changing the distance you cover in a given amount of time ... so that you cover
more distance in each successive time interval - accelerating.
When you jump out of an airplane you descend toward the earth. In fact, as you descend, you accelerate toward the earth. Assuming you pull the rip-chord to deploy the parachute before you reach "terminal velocity," you change your downward acceleration. You continue down. However, your rate of speed toward earth is slower. You have changed your descent rate (remember, any change in "rate" or "speed" is called "acceleration") and therefore you have experienced an "acceleration;" an "acceleration in the negative direction," but you never "climbed," you only changed the rate at which you were descending.