Which is more efficient in stopping a bullet, small cubes or fine grains of sand Recently, I was presented with the following problem, relating to pressure: which would be more efficient in stopping a bullet, small cubes or fine grains of sand.
Using my intuition, I would say the small cubes, since it has a larger surface area as compared to the fine grains of sand, and would likely have a strong structural integrity that is difficult for the bullet to penetrate.  I tested this out by punching my Ricola candies (cuboid shaped) as opposed to the sand on the beach, and I discovered that my hypothesis was correct to a certain extent, since it was much more painful punching the candies.
I could not therefore understand why in the war, sandbags are used to block bullets?  My teacher also disagreed with me by raising the fact that sand is highly compact and it is thus very hard for a bullet to penetrate.  I would like to ask, is my idea correct?
 A: When you hit sand, the sand has time to move out of the way. It behaves like a fluid in that regard - dipping your finger in water isn't painful. But doing a belly-flop off the high dive into the water is extremely painful, the reason being that the water has no time to get out of the way, so it behaves more like a solid.
Sand is the same way. When a bullet hits sand, the sand has no time to clear out of the way of the bullet, so it takes the brunt of the impact and behaves like a solid. Sandbags are therefore somewhat useful against bullets - the problem is that after a few rounds, sand starts to leak out of the bag. Sand's greatest utility is the fact that it is cheap and abundant, and it doesn't take a long time to place down sandbags in front of your position. 
It is also notable that sand is essentially a bunch of small cubes. Your candies won't stop a bullet because they are too big - they won't be able to compact against each other like sand can, and thus the bullet will just pass right through. 
