# Deceleration problem

1. May 5, 2007

### BigAl

I posted a question as thus :

A train moving at 100 km/hr uniformly decelerates at a rate of 10 km/hr each hr. How far does it travel before stopping ?

A few people interpreted the wording of the question to mean the train travels through each hr at a constant speed and then on the cusp of the hr instanteously decelerates by 10 km /hr .

Their rational is that the stated deceleration of 10 km/hr 'each' hr somehow contadicts the statement uniformly decelerates.

They argue the question is ambiguous .

Do they have a point . Or are they misreading the question.

2. May 5, 2007

### Hootenanny

Staff Emeritus
To me, the phrase "uniformly decelerates" would suggest that the rate of deceleration would be constant at 10km/hr2; "uniformly decelerates" removes all ambiguity. However, you may wish to consider rewording your question thus;

A train moving at 100 km/hr uniformly decelerates at a constant rate of 10 km/hr2. How far does it travel before stopping?

Here there is no 'wriggle' room whatsoever. That said, I think that someone has to work really hard to misinterpret your original question, I'd just tell them to sit down and shut up.

Last edited: May 5, 2007
3. May 5, 2007

### MrMaxus

Let them interprete "If I walk, I walk 6 kilometers each hour"
Do they really think you stand still and then run super fast to make the 6km in a second?

4. May 5, 2007

### BigAl

Thanks for the responses guys .
Here's another comment on the question
"My math sense is more than adequate, thanks. A's in Calc and Diffy Q. Your questions, on the other hand, are very poorly written and ambiguous. It's one thing to pose a poorly phrased question, it's another to attack those who point out how poorly phrased it is. "

This from a University of Michigan graduate engineer.

Is this a fair comment on the question ?

5. May 5, 2007

### ZapperZ

Staff Emeritus
I agree with Hootenanny whereby you should have phrased the question the way he suggested. However, I also think that since you DID phrased it in a rather strange way, someone like me would then start to think "Now there must be a reason he didn't ask it directly. So this isn't the simple question that we are all familiar with".

So if you are giving this to students who are familiar with acceleration being L/T^2, and you are now asking something with L/T per T, I can fully understand if the students start thinking that this question is more difficult than the usual type.

So yes, your question is ambiguous.

Zz.

6. May 5, 2007

### BigAl

ZZ ,

But ambiguous in what way . The alternate conclusion was this step wise deceleration assumption. Which by the way would be physically impossible for a train to do .
Can one assume this step wise deceleration and satisfy a description of uniform deceleration ? The decleration might have been stated in a different manner than the standard / t squared but is this enough to overtake the uniform assumption ? I think not.

7. May 5, 2007

### Hootenanny

Staff Emeritus
I agree with you somewhat Zz, if I was faced with the question I would have probably pondered for a while, but my common sense would suggest that the question 'intended' a constant acceleration. However, I can equally see how the problem could have been interpreted as an instantaneous acceleration every hour. So, yes the question could have been phrased better and perhaps a little ambiguous; but still, I would have answered the question the way it was intended.

8. May 5, 2007

### ZapperZ

Staff Emeritus
But that is what would be puzzling. Why word it in such a strange way, when the standard way would have been a lot simpler, clearer, shorter, and direct? I certainly would have asked that in my head.

When you word it in an unfamiliar way, then you open it up to varying ways to interpret it because the "rules" haven't been agreed upon by all the "players". If you had given the acceleration as L/T^2, do you think such questions would have come up? Nope. Because there's no other way to interpret it because everyone who has done these types of problem would know clearly what it means exactly. The fact that it wasn't asked that way is enough to trigger the notion that this isn't meant to be a "standard" question.

Zz.