# Physics question! Plz answerr! VERY IMPORTANT

gotchaeme | 23:11 Fri 12th Jun 2009 | Science
One species of dinosaur had a mass of about 1500 kg, as it walked, could support its whole weight on a single leg bone of length 75cm. If the breaking stress of the bone is 1.8 x 10(the 10 is to the power of 7) Pa then the minimum diameter the leg could have been to support the animal without fracture is?

I need the answer and ALL the workings out....
It is very important and is needed ASAP
Thankss Btw its either a,b,c or d

(a)1.6cm
(b)2.8cm
(c)3.2cm
(d)4.8cm

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No best answer has yet been selected by gotchaeme. Once a best answer has been selected, it will be shown here.

why don't you first tell us which answer you think it is, and why?
bit of lengthy question for a brontosaurus
Pressure = Force / Area
Area of circle (assuming circular bone) = pi x radius squared

If you can't work it out from that, I'll try and help a bit more
I was about to say that ↑
E=Mc 2
that was very pretty, zig.zag :o)
Question Author
I have no idea what to do....I need told how to do it and the answer.....i have loads of coursework due in tomorrow and i have been stuck on this for AGES
your homework is due in on a Saturday? that's harsh..
Question Author
Well its coursework and yeahh i know
because this is my last chance for correctss
and this is one of my last questionss
if pressure = force/area

then area = force/pressure = weight/pressure
= (1500 x 10)/18000000
= 8.33 x 10^(-4) metres squared

using 10 metres/second squared as acceleration due to gravity

area = pi x r x r
r = sqrt(area/pi) = sqrt(2.65) = 0.016 m, i.e 1.6 cm

The length of the leg bone doesn't actually matter here, as the weight is assumed to be acting straight downwards (and the bone is assumed cylindrical like a pipe), so no matter whether it is short or long the breaking stress is the same (stress = force/area in Youngs Modulus (stress/strain where strain = extension/original length)).

It matter to the dinosaur though, as it could be a long way to fall or a long distance along which blood has to be pumped!

Hope that makes some sense

Question Author
Thank you =]

But my teacher gave me a way of working out,
He wrote,
stress = force/area Area = Pi r squared

1.8 x 10.7 = 1500 x 9.8
---------------
Pi r squared

He then told me to find r...?!
Question Author

1.8 x 10.7 = 1500 x 9.8/Pi r squared **
that's exactly what I've done too!

pressure = force/area = weight/(pi x r squared)

r squared = weight/(pi x pressure)
r = sqrt(weight/(pi x pressure))

It may be easier for you step by step. First calculate the area as a numerical number as you have been given the inputs to do so. Then you know that this area = pi x r squared.

so area/pi = r squared
r = squareroot of (area/pi)
Question Author
Are you good at physics, i have a few more questions haha
I'm ok, I'll help if I can.

Was my other answer explained ok, it's really hard writing maths down in a satisfactory way!
gotchaeme it would be soo funny if Pigletion turns out to be your physics teacher!
LOL, I would say that Pigletion is either good at physics or has an engineering degree :)

Nice answer Pigletion, I started to work it out and then realised it's been over 20 years since I've had to do this stuff!
Question Author
Well im gratefull,
didnt really understand the steps haha
but thanks anyway
The other questions involve me showin u a picture..
always try and break the problem down into its simplest form and constituent parts. Write down what you know from the question, this often helps you to remember a formula in an exam, then try to logically reach your answer.

Examining units is also a good idea in physics; if you know the units of the answer, you can often work out what has to be multiplied by what (or whatever) to reach it.

A diagram is always good too!

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