You are Here: Home >< Maths

1. ----
2. (Original post by PerigeeApogee)
Hello,

So I'm getting frustrated here.}\text{ I've got a problem where the answer is an angle.} \text{ The final answer is given by the exam solutions as a degree.

However, when I perform the working with all variables in degress, I get the wrong answer, and only get the right answer when I perform the working in radians and then convert to degrees at the end. Clearly, there's something amiss here.

It's probably a school boy error. But here it is:

So, like I said, working in radians then converting works. Working in degrees throughout doesn't.

What's the problem here?
Well I haven't checked but it looks like you have rounded and then used your rounded figure in future calculations.
3. One problem I can see is that is a length. You then multiply it by an angular speed Q to get a speed (or angular speed, I don't know). But the important thing is that has to have units m/s because you then add it to W which has units m/s. This can only make sense if Q is in radians.
4. (Original post by Mr M)
Well I haven't checked but it looks like you have rounded and then used your rounded figure in future calculations.
Chaos theory?
5. I'm, guessing here, but in your formula, Q is supposed to be in radians/s.
If you convert it to degrees/s, you are making it 57.3 times bigger. But nothing else in your formula is factored upwards. So it won't give a valid result.
6. (Original post by PerigeeApogee)
How?

Radians do not have dimensions, neither do degrees. They're just ratios of lengths.

Q is an angular rate, so its units are regardless of whether you measure it in radians or degrees per second. So it should work either way, no?
Radians per second and degrees per second are different units.

Radians have the nice property that if you have an angle in radians and you multiply it by a length then you get the length of an arc. If you try this sort of calculation with degrees then you don't get a length at the end, you just get something else. That's why when doing this sort of calculation, you need to use radians.
7. (Original post by PerigeeApogee)
But presumably whatever you do on the RHS will ultimately be fixed when you perform the inverse tangent later on.

i.e., you might factor it upwards by 57 when you use degrees, but since you perform the inverse tangent with your calculator in degrees mode, this is equivalent to keeping Q in radians/s and performing inverse tangent with your calculator in radians mode. No?
I wasn't meaning use of degrees or rads in your calculator. You would just use a different mode there.

Suppose you have the expression:
Now let W=U=100 and Q = 1 radian, then

Now use degrees,

Vastly diffent results depending on whether the angle on the numerator and denominator is in degrees or rads.

I think that's the kind of thing that you have done in your own calculation.
8. (Original post by PerigeeApogee)
Where did I round?
All the two decimal places stuff (unless you couldn't be bothered to type the whole calculation into this thread).

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: December 4, 2010
Today on TSR

### Oxford interview invitations

When can you expect yours?

### Official Cambridge interview invite list

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.