Question

why we fill hard walk on sand?

Answer 1

due to frictional force

Answer 2

The main reason it is difficult to walk on sand is because the sand grains are un consolidated (loose). So with each step you take, you just move the sand grains around (similar to the plastic ball pits that children play in).In contrast, if you walk on a hard sandstone, the grains are cemented and create a hard surface, so if you walk on solid rock, the grains mostly stay in place. You have a much more stable surface. If you are walking barefoot, you have some more difficulties:
1.Much of sand is made up of broken quartz, which can be quite abrasive on your skin.
2.If the weather is hot, the sand can feel very hot!