# Engineering - Support Reactions

I thought I’d been getting the hang of these but I’m slightly confused about this example. Where has the (L/4) come from? I get the first part: moment about R1 = (R2 x L) + (-P x L/2) + (-w x L/2) = 0 but not sure where that last L/4 has come from. Could anyone help?
(edited 7 months ago)
Do you have the question?
(edited 7 months ago)
Original post by mqb2766
Do you have the question?

There’s not really a question, it’s just an example of how to find R1 and R2 in terms of P, w and L. Attached the slide in my OP.
Original post by kwikmaffs
There’s not really a question, it’s just an example of how to find R1 and R2 in terms of P, w and L. Attached the slide in my OP.

THere was nothing before defining P and w? What did it do for moments about R2?
Gut feeling is its a typo but the "w/unit length" isnt very clear on the diagram.
Original post by mqb2766
THere was nothing before defining P and w? What did it do for moments about R2?
Gut feeling is its a typo but the "w/unit length" isnt very clear on the diagram.

Nope. For moments about R2 it did:
-R1 x L + P x (L/2) + w(L/2) x (L/4 + L/2) = 0.

But if I was to do it the way I thought was right I’d do moments about 2:
(R1 x L) + (-P x L/2) + (-w x L/2) = 0.
So for R2 again I’m not sure where they’ve got that last bit of (L/4 + L/2).

Honestly the slides for these lectures are so bad, there’s just random numbers everywhere with no explanation of where they came from (and they’re badly formatted too lmao).
Original post by kwikmaffs
Nope. For moments about R2 it did:
-R1 x L + P x (L/2) + w(L/2) x (L/4 + L/2) = 0.

But if I was to do it the way I thought was right I’d do moments about 2:
(R1 x L) + (-P x L/2) + (-w x L/2) = 0.
So for R2 again I’m not sure where they’ve got that last bit of (L/4 + L/2).

Honestly the slides for these lectures are so bad, there’s just random numbers everywhere with no explanation of where they came from (and they’re badly formatted too lmao).

If w was the weight of a uniform beam Id agree with you. However w seems to be something related to weight per unit length so wL would be the weight. However, that doesnt explain the L/4 and 3L/4 (though they do sum to L). Id really need to properly understand how w (and P) were defined.
(edited 7 months ago)
Original post by mqb2766
If w was the weight of a uniform beam Id agree with you. However w seems to be something related to weight per unit length so wL would be the weight. However, that doesnt explain the L/4 and 3L/4 (though they do sum to L). Id really need to properly understand how w (and P) were defined.

Okay so in another PowerPoint I found a diagram and it says P = a point load and w = a distributed load (load/unit length).
Original post by kwikmaffs
Okay so in another PowerPoint I found a diagram and it says P = a point load and w = a distributed load (load/unit length).

That makes a bit more sense. As its not symmetric, there must be something that defines w somewhere so the reason why its 1/4 about one support and 3/4 about the other.
Original post by kwikmaffs
Okay so in another PowerPoint I found a diagram and it says P = a point load and w = a distributed load (load/unit length).

Actually looking at the original diagram, w only applies (uniform) to the first 1/2 of the length so the load is wL/2 and the com is L/4 from R1 and 3L/4 from R2. Its zero in the second 1/2 of the length.
(edited 7 months ago)
Original post by mqb2766
Actually looking at the original diagram, w only applies (uniform) to the first 1/2 of the length so the load is wL/2 and the com is L/4 from R1 and 3L/4 from R2. Its zero in the second 1/2 of the length.

So the L/4 and 3L/4 have come from the mid point of where the force is acting? Is that right or am I being thick?
Original post by kwikmaffs
So the L/4 and 3L/4 have come from the mid point of where the force is acting? Is that right or am I being thick?

THats right. There is a uniform force/load acting from 0..L/2 (on the left) so that means it acts like a point force/load of wL/2 at L/4 from the left and 3L/4 from the right.
(edited 7 months ago)
Original post by mqb2766
THats right. There is a uniform force/load acting from 0..L/2 (on the left) so that means it acts like a point force/load at L/4 from the left and 3L/4 from the right.

Okay thanks, it’s clicked now! Makes sense. Appreciate it
Original post by kwikmaffs
Okay thanks, it’s clicked now! Makes sense. Appreciate it

From a quick google, it is a reasonably standard notation for a distributed load, but not something Id really come across.