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I have a hammock that I want to hang using a tree on one end and a wooden post set in a bucket of cement on the other end.

How would I determine how heavy the cement needs to be so that it would support the weight of a 250lbs person in the hammock?

Any ideas would be appreciated.

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It would be better to search for tried-and-true hammock hanging schemes. –  Yrogirg Aug 11 '12 at 5:54

2 Answers 2

up vote 2 down vote accepted
    ^       |\                   /
    |       |α\                 /
    |       |  \_______________/
    |       |
 H  |       |
    |  _____|_____
    |  |         |
    |  |    M    |
    V  |_________|

            <---->
               b

The figure should explain the notions (except for $T$, but never mind on it). $m$ is your mass, $M$ is the mass of the block.

$$T \cos \alpha = \frac{1}{2} m g$$ $$T H \sin \alpha = M g b$$

Thus the answer for the minimum mass required for the concrete block is

$$M = \frac{1}{2} m \frac{H}{b} \tan(\alpha)$$

But this won't do since it the low boundary, so multiply it by 5-10 and bury the bucket.


To community: Maybe we have a policy on such questions?

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+1 for the remark about the safety factor. I think the SE Physics Meta is the best place to discuss proper policy for questions like these –  Rody Oldenhuis Aug 11 '12 at 9:00
    
My comment is those question enhance the pratical skill of the student and encourage them to "do it yourslf" –  Victor Aug 12 '12 at 2:51

Depends what angle the lines on the hammock will be at or how tout it is. The lower the angle from the post the line is, the less mass you will need. A wider base will also mean you will require less mass while a taller pole for mounting the hammock (due to levers) will mean you would require more mass in concrete. A formula could be developed that would calculate this and take into account all the factors however you can just bury the concrete block or set set metal fish eye loops or hooks in the concrete and use camping pegs (at a 45 degree angle away from the middle of the hammock where the mass will need to be held) to help hold it down.

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