One of the Example in my Textbook is

  • A body travels a distance of 2 m in 2 seconds and 2.2 m in next 4 seconds. What will be the velocity of the body at the end of 7th second from the start.

  • In Solution they let 'a' be the uniform velocity and used equations of motion (which we use when there is an uniform acceleration motion) and solved it.

  • My Doubt is How did they let 'a' be the uniform acceleration.

  • I would have taken different acceleration for 1st 2 seconds , next 4 seconds and the last one second.

  • Either the question is incomplete (i.e., It should provide it is an Uniform Accleration Motion) or I am Wrong Somewhere.

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    $\begingroup$ I agree that they should have told you that the acceleration is uniform. It's normally a safe assumption if you're doing exercises like this, but that doesn't excuse the question setter $\endgroup$ – Dr Xorile Jul 4 '15 at 3:06
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    $\begingroup$ Yes, the question is incomplete. Either based on context the acceleration was supposed to be assumed constant, or else the question setter goofed up. $\endgroup$ – Cicero Jul 4 '15 at 3:07

You conclude that constant acceleration is appropriate for one of a few reasons:

  1. You read in the question text that you should treat it that way.

  2. If (1) does not apply, you read in the question text that some physical situation obtains and you know or suspect that this situation is usually well represented by a constant acceleration.

  3. If neither (1) nor (2) applies, you reason that you have recently done a constant acceleration unit in class or read that chapter in the text and decide that this is what the author of the question meant. This is most reasonable when the question comes from the back of the chapter. You should probably write explicitly that you are making this assumption.

  4. If none of (1), (2) and (3) apply, you decide to make this assumption. You should probably write explicitly that you are making this assumption.

Frankly the author of the question should insure that either (1) or (2) are true if this is a basic learning by doing repetition task or an exam question.

In some open-ended questions that are intended to resemble problems encountered in the real world, you will generally find yourself using (3) or perhaps (4) and you should be prepared to defend the choice.


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