force due to a slab?

the force on a mass on top of a slab

slightly confused as to what you mean by that... can you give us more information about the question?

basically imagine i have a slab of a bed or something...think 20km + in scale.

So I have a giant slab under the surface of the earth, and within this slab of density a, i have two spheres of density b st b>a. I'm trying to find out how i can work out the gravity anomaly due to these spheres of higher density rock in the slab. I just can't think how I would go about this without knowing the mass of the slab

Since the slab thing (apart from where the sphere is), is the same before and after, can i just ignore them and compare the force due to two spheres of density a to two spheres of density b?

I hope this is clearer.

basically imagine i have a slab of a bed or something...think 20km + in scale.

So I have a giant slab under the surface of the earth, and within this slab of density a, i have two spheres of density b st b>a. I'm trying to find out how i can work out the gravity anomaly due to these spheres of higher density rock in the slab. I just can't think how I would go about this without knowing the mass of the slab

Since the slab thing (apart from where the sphere is), is the same before and after, can i just ignore them and compare the force due to two spheres of density a to two spheres of density b?

I hope this is clearer.

What exactly do you mean by "gravity anomaly"?

Are you asking about the force of attraction these two spheres have for each other? If so, you need to know their mass and how far apart they are. You know neither of these.

I'm also intrigued as to what all this is about. What are you trying to do?

it's a geophysics problem i'm working on. imagine you have a cuboid with two small marbles in it - treat this as a model to what i'm doing.

Anyhow the cuboid has density a and marble has density b. I odn't know the size of the cuboid, but i know the size of the marbles.

So using this info I have to work out the gravity anomaly. This basically means I need to find the gravitational difference between the case where I assume there are no marbles in the cuboid, and the case where there are two marbles in the cuboid of a density higher than the surrounding cuboid.

Bit of a mind **** tbh. I am assuming a volume for the slab/cuboid and calculating the anomolies this way and i'm going to compare it to my original method of using the sphere density of b-a and working out the gravitational anomaly using this.

That's a bit clearer, but I still don't know what you mean by "gravitational difference"? Are we talking about the difference in weight between the slab with and slab without the marbles? If not, what other "difference" do you mean?
If it's the weight difference, then you just need to subtract the weight of the two spheres of slab material (g times density of slab times volume of marbles) and then add the weight of the 2 marbles (g times density of marbles time volume of marbles)
If S is the initial weight of the slab, the new weight with the marbles is
S - 2gVdₒ + 2gVdₓ
V is volume of a marble, dₒ is density of slab, dₓ is density of marble.
Or S + 2gV(dₓ -dₒ )