Solving a Physics Problem: Is My Calculation Correct?

In summary, the problem is that the beam does not obey Hookes Law- it is beyond its elastic range in the outer fibers and in a semi-plastic state. The errors in your calculations were due to incorrect conversions from mm to meters and the use of US units in your analysis.
  • #1
jubaitca
4
0
have uploaded the question on the web. Please have a look at:

http://amd.streamload.com/jubaitca/Hosted/pictur28.JPG

I have tried doing this problem, but i am not getting the right answer. This is what i did:

y(unknown)=40cos60 = 7 mm
x (unknown) = 40 cos 30 - 20 == 14.641mm

Then

Pcompression = 2(75x7x(34.641/2)) + 2(((20x75)/2)(14.641)
= 40149 N

But the answer is somehow... 73.92 KN

Is there anything wrong with my calculations?

Thanks
 
Last edited by a moderator:
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  • #2
jubaitca said:
have uploaded the question on the web. Please have a look at:

http://amd.streamload.com/jubaitca/Hosted/pictur28.JPG

I have tried doing this problem, but i am not getting the right answer. This is what i did:

y(unknown)=40cos60 = 7 mm
x (unknown) = 40 cos 30 - 20 == 14.641mm

Then

Pcompression = 2(75x7x(34.641/2)) + 2(((20x75)/2)(14.641)
= 40149 N

But the answer is somehow... 73.92 KN

Is there anything wrong with my calculations?

Thanks
You might note first off that this beam does not obey Hookes Law, that is, it is beyond its elastic range in the outer fibers and in a semi-plastic state, ( or else the stress distribution would vary linearly rather than uniformly over the top section). I think the problem may have been written that way to make the calculations less tedious. Anyway, you have proceeded correctly, but have made 2 math errors. 40(cos60) is 20 (not 7!). And your conversions are off: 75MPa is 75*10^6 N/m^2, and you should convert the mm distances to meters (I'm glad i work in psi and inches!). Once you make those corrections, you will get the correct compressive (and tension) force of 73.92kN. To get the moment , you've got to find the c.g. of the tension and compression load, then determine the couple.
 
Last edited by a moderator:
  • #3
thanks a lot...im so stupid at times...by the way, i don't need to convert it to meters because MPa = N/mm^2.

but thanks a lot, i have wasted so many hours thinking on this and the error just was that i could use my calculator properly...lol
 
  • #4
jubaitca said:
thanks a lot...im so stupid at times...by the way, i don't need to convert it to meters because MPa = N/mm^2.

but thanks a lot, i have wasted so many hours thinking on this and the error just was that i could use my calculator properly...lol
Yes, thanks, I'm so used to USA units that metric leaves me cold. The use of SI in beam anaysis and design in the States is virtually non existent, and seing as how attempts over the past 40 years to convert have gone nowhere, I suspect that it always will be.
 

1. How do I know if my calculation is correct?

One way to check if your calculation is correct is by double-checking your work. Make sure you have used the correct formulas and units, and that you have not made any simple arithmetic errors. You can also ask for help from a teacher or peer to review your work and provide feedback.

2. Can I use different methods to solve a physics problem?

Yes, there are often multiple ways to solve a physics problem. It is important to choose the method that you are most comfortable with and that will give you the most accurate and efficient solution.

3. Should I show all of my work when solving a physics problem?

Yes, it is important to show all of your work when solving a physics problem. This not only helps you keep track of your steps, but it also allows others to understand your thought process and provide feedback on any mistakes or areas for improvement.

4. What should I do if my calculation does not match the expected answer?

If your calculation does not match the expected answer, double-check your work to ensure you have used the correct formulas and units. You can also try solving the problem using a different method or asking for help from a teacher or peer.

5. Can I use a calculator when solving a physics problem?

Yes, calculators can be a helpful tool when solving a physics problem. However, it is important to understand how to use the calculator properly and to not solely rely on it for your calculations. Additionally, make sure to include the appropriate units in your final answer when using a calculator.

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