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1. Centre of Mass
A fishing rod is made of three parts clipped together, each uniform and of length 1.2 metres. The separate parts have mass 60grams, 40grams and 25grams. Half of the heaviest part is wrapped in a sleeve to which a reel is attached, with total mass 75grams and centre of mass 40cm from the end of the rod. Find the distance of the centre of mass of the whole rod from this end.

The ball at the end represents the centre of mass of the reel.

So listing the masses in terms of kg rather than grams, I had the following data:
, , giving me , but apparantly the answer is , and I don't see how to get that.
2. Re: Centre of Mass
I get the same result as you.
3. Re: Centre of Mass
(Original post by ghostwalker)
I get the same result as you.
Okay, that's reassuring. Would you (or anyone else) mind having a look at a similar question?

A badminton racket consists of a frame 28cm long attached to a long shaft 38cm. For the last 16cm of its length the shaft is surrounded by the grip. If the frame, the shaft and the grip have mass 70grams, 40grams and 30grams respectively, and the centre of mass of each part is at its geometrical centre, find the distance of the centre of mass of the racket from the end of the shaft.

My calculations look like , , but my textbook is giving me an answer of .

Thanks for any help.
4. Re: Centre of Mass
Yeah I got the same as you in both
5. Re: Centre of Mass
(Original post by Julii92)
My calculations look like , , but my textbook is giving me an answer of .

Thanks for any help.
I get 33.1 to 3 sig.fig.

Your 0.04g should be multiplied by 19, not 27.
6. Re: Centre of Mass
(Original post by ghostwalker)
I get 33.1 to 3 sig.fig.

Your 0.04g should be multiplied by 19, not 27.
Thanks man, I'll have a look at that in the morning. I really need to start paying you for this
7. Re: Centre of Mass
(Original post by Julii92)
A fishing rod is made of three parts clipped together, each uniform and of length 1.2 metres. The separate parts have mass 60grams, 40grams and 25grams. Half of the heaviest part is wrapped in a sleeve to which a reel is attached, with total mass 75grams and centre of mass 40cm from the end of the rod. Find the distance of the centre of mass of the whole rod from this end.

The ball at the end represents the centre of mass of the reel.

So listing the masses in terms of kg rather than grams, I had the following data:
, , giving me , but apparantly the answer is , and I don't see how to get that.
You have a stray - sign in the final term for some reason. The sleeve, which is wrapped around the first 60 cm of the first section, produces a moment in the same direction as all the others, no? With this correction you get the quoted answer.

When the question says "40 cm from the end of the rod", it must mean "towards the centre of the rod", rather than in the direction that you seem to have assumed. That's because the sleeve is wrapped around the end section, rather than poking out from it.
8. Re: Centre of Mass
(Original post by atsruser)
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Yes, you're correct, I was going by the diagram, assuming it was based on the book, rather than the wording.

Good spot. +rep.