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M1 moments on a log question help !!!!!

hi, I really need help with part c as I have managed to construct one equation but I don't get where the other equation comes from .

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. The other equation that I need has to include 1500n from part a. However isn't this a new model though in part c . Please explain this. Thanks.

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Reply 1

coconut64

Where does this first equation come from and how do you know you need to go back to part a to solve this ?

Reply 2

Middriver

Original post by coconut64

Where does this first equation come from and how do you know you need to go back to part a to solve this ?

With your second equation you have 2 unknowns w&x so you need to substitute another equation, so you just take moments about point d(tipping),you don't use the equation from a it's no longer being considered a uniform rod.

(edited 7 years ago)

Reply 3

coconut64

Original post by Middriver

Hi thanks for helping. I don't know which one is the second equation that you are talking about can you elaborate a little. I only managed to construct one equation using 1000N . Thanks.

Reply 4

ghostwalker

Original post by coconut64

Hi thanks for helping. I don't know which one is the second equation that you are talking about can you elaborate a little. I only managed to construct one equation using 1000N . Thanks.

The first equation comes from the original set up used in part a) when taking moments about C.

The equation you derive is different to that of part a) because the assumptions are different, but the set up is still valid. The log starts to tilt about C when a force of 1500N is applied as described. So take moments about C and you get the given equation. If you can't see it, draw a diagram, and have a go and see what you can do, then we should be able to see more clearly what issue you're having.

Reply 5

coconut64

Original post by ghostwalker

Hi, I get that equation 1 is from part a but in part c we are told that the log is now non uniform . However the log was uniform in part a. I don't get how the set up is still valid . Is it because it's still the same log? I have uploaded my working. Thanks.

Reply 6

Middriver

Original post by coconut64

It's not the same equation from a, you use the same information, applying a force of 1500 at A, makes it tilt about D, so you take moments about D but change the position where the weight acts(4-X)

(edited 7 years ago)

Reply 7

coconut64

Original post by Middriver

It's not the same equation from a, you use the same information, applying a force of 1500 at A, makes it tilt about D, so you take moments about D but change the position where the weight acts(4-X)

Oh okay but how do you know you need to consider 1500n rather than using 1000n and take moment at a different point ?? Thanks.

Reply 8

Middriver

Original post by coconut64

Oh okay but how do you know you need to consider 1500n rather than using 1000n and take moment at a different point ?? Thanks.

Because the log tips about d when you apply a force of 1500n at A regardless of where the Mass acts. You don't know where applying 1000n will cause it to tip about d.

Reply 9

coconut64

Original post by Middriver

Because the log tips about d when you apply a force of 1500n at A regardless of where the Mass acts. You don't know where applying 1000n will cause it to tip about d.

Oh thanks so much. So even though the modelling assumption is different this still works because it's the same log ? Is this what you mean by the same set up?