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# M1 moments question Watch

1. AB is a uniform rod of length 5m and weighs 20N. Find the magnitudes of C and D

I got 8N for D and 12N for C but apparently the answer is 8.6N and 12.4N

I did:
(1x20) = (2.5xd) so d = 8 and then I did 20-8 to get 12. What did I do wrong? thank you
2. (Original post by physicshelpme)
AB is a uniform rod of length 5m and weighs 20N. Find the magnitudes of C and D

I got 8N for D and 12N for C but apparently the answer is 8.6N and 12.4N

I did:
(1x20) = (2.5xd) so d = 8 and then I did 20-8 to get 12. What did I do wrong? thank you
3. (Original post by TenOfThem)
So do you think the answer in the back of the book is wrong??
4. (Original post by physicshelpme)
So do you think the answer in the back of the book is wrong??
If there is nothing more to the question than that diagram
5. (Original post by TenOfThem)
If there is nothing more to the question than that diagram
Nope, the full question is 'AB is a uniform rod of length 5m and weight 20N. In these diagrams AB is resting in a horizontal position on supports at C and D. In each case, find the magnitudes of the reactions at C and D' and then the diargram
6. (Original post by physicshelpme)
Nope, the full question is 'AB is a uniform rod of length 5m and weight 20N. In these diagrams AB is resting in a horizontal position on supports at C and D. In each case, find the magnitudes of the reactions at C and D' and then the diargram
Your book seems to think that 8.6 + 12.4 = 20, which should start alarm bells ringing!
7. (Original post by davros)
Your book seems to think that 8.6 + 12.4 = 20, which should start alarm bells ringing!
Good point

If the weight were 21N then their answers would be correct as 21/2.5 = 8.4
8. (Original post by davros)
Your book seems to think that 8.6 + 12.4 = 20, which should start alarm bells ringing!
Weird. I've looked over this question like 5 times in the past hour, wondering how they got those numbers, and until you pointed it out I too legitimately thought that 8.6+12.4 was equal to 20!
9. (Original post by aznkid66)
Weird. I've looked over this question like 5 times in the past hour, wondering how they got those numbers, and until you pointed it out I too legitimately thought that 8.6+12.4 was equal to 20!
Easily done! I started typing my previous post then went back to re-check my mental arithmetic in case I was about to make an idiot of myself

It's always worth a quick sanity check with these questions, especially if you suspect the book of having misprints.

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