# Homework Help: Acceleration vector exercise (coordinate system)

1. Nov 7, 2012

1. The problem statement, all variables and given/known data

All the formulas I was given are also on a separate attachment

I initially thought the answer was the last one because I did rf-ri and subtracted across and got 0 mi + 25mj but im not sure =[
1. The problem statement, all variables and given/known data

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2. Nov 7, 2012

### PeterO

rf-ri sounds like a good idea, but 0 mi + 25mj isn't the answer to that.

ri has a 50mi term
rf has a -50mi term

so when you subtract them the answer will not be zero [since they are different]

3. Nov 7, 2012

Oh thx for the much needed advice. I redid the problem and got the first choice as my answer which was from subtracting -25 from 25 and 125 from 100. Can anyone verify that choice a is correct =[]

4. Nov 7, 2012

### PeterO

My previous post should have said 25mi and -25mi - you can't look at "pictures" and respond at the same time.

While the first option is correct - the description of what you did is all backwards.

You actually subtracted 25 from -25, and subtracted 100 from 125.

5. Nov 7, 2012

Thanks a ton peter I understand now :)

6. Nov 7, 2012

### PeterO

By the way: applying the logic of multiple choice questions leads you to this answer.

If you consider just the i components.

If you add them [or subtract poorly], you get 0i [only one option has this].

If you subtract the second from the first you get 50i [only one option has this]

If you subtract the first from the second you get -50i [two options have this]

If the answer was not one of the -50i .... examples, you would not even have to consider the j component - possible but unlikely.

consider the j component. when you subtract here, you either get 25j or -25j

only one of those possibilities is offered - and there is your answer!!

Playing the multiple choice game rather than understanding the problem is not a recommended technique - but if all else has failed it is useful.
Remember: with a normal problem you are trying to find the answer; with a multiple choice question you are only trying to identify which of the options is the answer.