An explosion under a pile of timber and rocks

[Simon Peach]

There is a pile of rocks and timber, all the same weight, under it is an explosive device after the explosion the pile is ejected in the atmosphere, which timber or rock will land first?

 

[Baluncore]

The timber has a lower density, so a greater cross-section for the same weight, and will be accelerated faster than the rock. The timber will go higher and further, therefore it will return to the ground later. The rock will tend to land first.

If you empty a tip truck, or excavator bucket, that contains mixed timber and rock, the rock will tend to exit first, followed by the timber.

 

[kuruman]

There is a pile of rocks and timber, all the same weight, under it is an explosive device after the explosion the pile is ejected in the atmosphere, which timber or rock will land first?

Your description is vague and does not have a unique answer. To make a meaningful comparison, you have to specify what else is the same other than the weight (or mass.) A pile of mixed rocks and pieces of wood, all of the same mass, sitting on top of an explosive device will scatter in all different directions when the explosion occurs. Furthermore, any particular piece will have initial speed that depends on its share of the total energy released in the explosion.

As stated, your question has one answer, "the mass that has the smaller vertical component of velocity will land first because it spends less time in the air." That's ignoring air resistance.