Physics, help...anyone???

[blm]

This is not a hard dimensional analysis problem for someone who has some physics background, but I do not so it confuses me. Can anyone explain this in idiots terms so I can move on...?

The radius (r) of a circle inscribed in any triangle whose sides are a,b,c is given by: r=[(s-a)(s-b)(s-c)/2]*1/2, where s, is an abbreviation for (a+b+c)/2.

Is this dimensionally correct? Yes or no, please explain how you solved it.

NOTE: *1/2, symbolizes everything in the brackets raised to the 1/2 power.

[blm]

:dunno:

[Scrumhalf]

Not dimensionally correct.

(s-a), (s-b) and (s-c) all have dimensions of length, i.e. L.

So, (s-a)*(s-b)*(s-c) has the dimension of L^3 and the square root of (s-a)*(s-b)*(s-c) has the dimension of L^(3/2)

However, the left hand side of the equation has the dimension of L.

Therefore, the equation is dimensionally incorrect.

[IntensityX]

Yes Professor Gilligan is doing the Skipper and Ginger is doing a lesbian scene with Mary Ann :D

[blm]

Thanks...I told you it was simple, but we never discussed this in class.

[blm]

Can you help me w/2 more?

[Scrumhalf]

Sure, post 'em. It's been almost 15 years since my last physics class in grad school, so don't make it too complicated :)

[blm]

The approach was correct, but it the end result was not. Both sides end up with Length, therefore it is dimensionally correct, as per my professor. Not because I have any clue wtf I'm doing yet.

[K409G]

I'd have to agree with scrumhalf. You get a dimension with the power of 3/2 on the right and a power of 1 on the left. (L*L*L)^.5=L^3/2

[blm]

I agree but my prof made it seem like the important part was that both sides had a representation of length. However, mathmatically it can't be right (which I thought was the whole point) because as stated above, one side is equal to one and the other by a square root cubed or 3/2. Maybe he just messed up, he did look at it really quick. It doesn't help me much though if he is giving the wrong answers.

[Scrumhalf]

Yeah, your professor is full of it. I can't believe he would claim what he is claiming. It is as bad as saying 1 = 1.5.

[Scrumhalf]

By the way, the correct formula for the radius of a circle inscribed in a triangle with sides a, b and c is:

Radius = (1/s)* sqrt(s*(s-a)*(s-b)*(s-c))

where s = 0.5*(a+b+c)

As you can see, the quantity inside the square root is of dimension L^4 and the square root is therefore of dimension L^2. This divided by s which is dimension of L gives an overall dimension for the rght hand side of the equation of L, which is the same dimension as the left hand side of the equation.

[blm]

Originally posted by Scrumhalf
This divided by s which is dimension of L gives an overall dimension for the rght hand side of the equation of L, which is the same dimension as the left hand side of the equation.

This was my error and the reason the teacher was correct. I typed a 2 where there should have been an "s." I didn't even realize it, every time I looked at this post. :doh: