How to calculate the supporting force exerted by a stationary object (cupboard) and support (brackets) for it? I have a wall mounted Stuva Ikea cupboard, which measures 60x64x30cm. The base is 64x30cm. It is already supported by 2 holes at the back base, (64x60cm). 

But I need to add in L shape brackets underneath for extra support. Only, the brackets can support a max load of 15kg only. The cupboard is 15kg, I need to load another 15kg of books on, making the total 30 kg.
Can anyone help to calculate the distance that I have to put the brackets as additional support?
15kg max load brackets (28cm x 2cm)
base of cupboard (64x30cm)
total weight to support (30kg) - alr 2 holes wall mounted on base-to-wall (60x64cm)
Appreciate any help.
How do I calculate F=ma, as the cupboard and brackets are stationary?
@all who answered, more info here, until i figure out how to add images on here lol:
Cupboard frame 8.6kg
http://www.ikea.com/sg/en/catalog/products/70165163/
Cupboard doors 5kg
http://www.ikea.com/sg/en/catalog/products/90169103/
The cupboard backing board comes with 2 holes predrilled, and 2 plastic backing boards.the backing board is 2mm thick, doubt the screws i screwed in will actially hold it up. Ikea is no help at all! I hv two of this cupboards side by side (but not joined)
 A: I'm not 100% sure about this but here goes.
If your cupboard is in equilibrium, or not undergoing any changes in motion, the wall is pushing up on it with the same force it is pushing down on the screws/nails holding up the cupboard. That is to say $$ΣF = 0$$
Using: $$F = ma$$
and assuming you are near sea level, we can use the acceleration due to gravity at sea level in place of a. $$F=m(9.8m/s^2)$$ We can use the mass you mentioned $$F=(15Kg)(9.8m/s^2)$$ And since a Newton is 1Kg * m/s^2, we get $$F=147N$$
If it is already supported in two places then each one has about 73.5N on it. If you add another bracket(for a safety margin), it would be 49N on each one.
$$147N / 3\text{ Total brackets} = 49N/\text{Bracket}$$
Since the load and the cupboard and the books have the same mass, you can just double our force calculation. You would also double the number of brackets. $$(147N ⋅ 2) = (3\text{ Total brackets}) ⋅2$$
$$294N = 6\text{ Total brackets}$$
$$\text{Because:}$$
$$\frac{249N}{49N/\text{Bracket}}$$
$$\text{Cancel the Newtons, get the  Brackets out of the denominator and divide the fraction:}$$
$$6\text{ Brackets}$$
6 Brackets minus the two already in your cupboard, leaves you with 4 brackets to be evenly spaced along the bottom.
I hope this will help you decide how to mount your cupboard.
Edit:
Reading the comments above reminded me about the torque. If nothing else, I think a bracket at the top might stop the cupboard from leaning out from the wall/bending the brackets on the bottom.
Edit:
You could also technically just eyeball it, and say "Well, I add another 30Kg, I have brackets rated at 15Kg, so I'll add two more, plus one at the top to stop it from falling forward." However, I don't think this is a very safe approach, and would recommend adding another bracket on the bottom to your eyeball calculation as a safety margin, if you were to do this.
A: Put a 2x1 Woden bracket under the cupboard for support with several screws, much neater, cheaper and stronger job.
