Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Very Tough Question. Can you answer?

  1. Jun 26, 2010 #1
    Which one of the following is true for a deceleration?

    a)the velocity remains constant
    b)the acceleration is negative
    C)the acceleration is in the direction opposite to the velocity
    d)the acceleration is zero
  2. jcsd
  3. Jun 26, 2010 #2
    Very tough indeed... is this a serious question?
  4. Jun 26, 2010 #3
    yes, though the difficulty stated in the title was simply to attract competent physics people.
  5. Jun 26, 2010 #4


    User Avatar
    Science Advisor

    So, since this is homework, I guess the first question is "do you know what "acceleration means". That should remove one or two of the possible answers. And the second question is "do you know the difference between "acceleration" and "deceleration"?
  6. Jun 26, 2010 #5
    the answer is obviously C. But I was wonder what is so wrong with answer B.
  7. Jun 26, 2010 #6
    Because "negative" depends on your choice of coordinate system. Acceleration is a vector.
    "Negative" and "positive" when dealing with vectors really only shows direction, so something "decelerates" if the acceleration is opposite to its motion. However, what if the object is moving in the negative direction? Then the "deceleration" would be in the positive direction.

    Also, "deceleration" is more of a layman term. In physics there really is only "acceleration". Which can be positive or negative, again depending on your coordinate system.
  8. Jun 26, 2010 #7
    I know the solution of this question.
  9. Jun 26, 2010 #8
    but if an object is accelerating in the negative direction, then deceleration would make the acceleration negative.

    If something was accelerating at -9.8m/s^2, deceleration would mean the negative of the acceleration so it would go in the positive direction. So isn't that the same as the direct opposite to the velocity?
  10. Jun 26, 2010 #9
    I have no idea what you are talking about.
  11. Jun 27, 2010 #10
    You seem to be thinking of deceleration as an operator on the acceleration vector, it isn't. It is just a relationship between velocity and acceleration. An object whose velocity vector is pointing in one direction and is undergoing a net acceleration in the opposite direction is said to be decelerating.

    No, deceleration would mean the object is slowing down, nothing else. There's a deceleration if and only if the magnitude of the velocity vector is decreasing. If someone says a car is decelerating, you have no idea if its acceleration is negative or positive or in the process of changing signs, all you know is that there is an acceleration that is slowing the car down, and this information is independent of the fact that it was accelerating at -9.8ms/^2 previously.
  12. Jun 27, 2010 #11

    So essentially what you are saying is that acceleration can fall under the terms deceleration and if an object traveling at 40m/s has an "acceleration" of -1m/s^2, the so called "the acceleration is negative" would actually make the -1m/s^2 a positive value and therefore would be wrong?

    So this is all a matter of the way a word is used?
  13. Jun 28, 2010 #12
    Please access this website and observe the multimedia.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook