# Homework Help: Messed up joules and newtons in an exam

1. Apr 3, 2012

### valmancer

I know this is just a piece of whining, but I feel awful about it and want other's opinions. It wouldn't really bother if I didn't think I'm OK in physics.

In a question I had to calculate potential energy and naturally wrote the answer down as joules. However, when I came back to check the question I wasn't as sure and with some kind of twisted logic concluded that the answer should be in Newtons.

The question here is: how much of the points do you think the teacher will take off? The first part is correct until the units of the final answer, but in the second part I go with the assumption that I have a value for force on my hands.

I expect no-one to care, but it'd be nice to hear some opinions.

2. Apr 3, 2012

### phoenix:\\

If you did the calculations correct but messed up on the units you should still get some partial credit. It is dependent on the instructor as well though, but in my case with unit mishaps but with the correct or logical process, a 10 point question = 6 points.
If you don't like the points, just talk to your professor about it, in my same case, they often will give the student some more points or all the points if they show an interest enough in that they do know how to solve the problem but had some anxiety issues.

3. Apr 3, 2012

### Ouabache

You may want to visit this post. It clarifies the concepts of Joules and Newtons.
As you may (or perhaps may not know). One is a unit of energy and the other, a unit of force.
So you cannot simply scale one by a constant to obtain the other. You may get some
partial credit on the first part, but after you changed to Newtons, I would not expect much.

When an error of similar concern occurred on my exams, I just bit the bullet
and tried to remember to be more careful next time. In one physics class in particular,
I recall them mixing units within the question, just to make sure we understood how
to convert to the units required in the solution. Dimensional analysis is useful technique
to resolve this.

Last edited: Apr 4, 2012
4. Apr 3, 2012

### castro94

i would call that a good mistake , if you did everything else right , and just messed up on the units , it is just a good mistake , we all do it sometime , as long as you got the concepts right , then you shouldnt care to much , thats what often bothers me with exams and all that "testing education " it often only show how good you are at memorazing alot of words and units. to me the most important thing is that i myself feel that i have understood . just learn from your mistake , and make sure you get it right next time :)

5. Apr 3, 2012

6. Apr 3, 2012

### jim hardy

My friends here in Ozarks are highly intelligent laypeople not enamored of the metric system.
So when they asked me "What's a Newton " i tried to paint a mental picture that'd mean something. So i replied "It's about the weight of a handful( six or seven) of Nabisco Fig Newtons. Roughly 1/4 pound . More precisely a kilogram weighs 9.8 Newtons but who would remember that? Just remember it's a force and think of those figbars."

seems to have stuck.

old jim

7. Apr 3, 2012

### lisab

Staff Emeritus
In my experience, profs tend to be veeeery picky about units. Simple math errors? Meh, they're nothing. But a unit error could indicate a lack of understanding, so profs can be pretty harsh on grading them.

Just put this one behind you, keep on keeping on.

8. Apr 4, 2012

### BobG

So, I take it you don't use the units as a double check to make sure your calculation at least had a chance of being correct? Just tacking the units on at the end means the units will be of no help to you at all. Hopefully, the units used for the variables in the problem all work out to the units you're looking for.

On the bright side, at least you didn't put Newton-meters for your units. That would definitely show a lack of understanding.

9. Apr 4, 2012

### Ouabache

For those not familiar with the Mars Climate Orbiter incident. The spacecraft approached
Mars at an improper low altitude, causing it to enter the upper atmosphere and disintegrate.
The problem was due to inconsistent use of units by the people controlling the orbiter.

10. Apr 4, 2012

### jim hardy

Darn those continentals - they gave us metric bolts and lead-free solder too.