Why do your feet sinks into sand when you are on toes rather that flat feet?

When you stand on your toes, the weight of your body is concentrated on a smaller surface area, which increases the pressure exerted on the sand. This causes the sand to deform and flow away from your feet, creating a sinkhole. In contrast, when you stand with your feet flat on the sand, the weight of your body is distributed over a larger surface area, which reduces the pressure exerted on the sand. This prevents the sand from deforming and flowing away, so your feet don't sink in.

Here's a more detailed explanation of the physics involved:

* Pressure: Pressure is defined as the force applied to a surface per unit area. In this case, the force is the weight of your body, and the surface area is the area of contact between your feet and the sand.

* Stress: Stress is defined as the internal force per unit area that resists deformation. In this case, the stress is the force that the sand exerts on itself to resist being deformed.

* Shear strength: The shear strength of a material is the maximum stress that it can withstand before it starts to flow. In this case, the shear strength of the sand is the maximum stress that it can withstand before it starts to deform and flow away from your feet.

When you stand on your toes, the pressure on the sand is increased, which exceeds the shear strength of the sand. This causes the sand to deform and flow away from your feet, creating a sinkhole. In contrast, when you stand with your feet flat on the sand, the pressure on the sand is reduced, which is below the shear strength of the sand. This prevents the sand from deforming and flowing away, so your feet don't sink in.

The same principles apply to other soft surfaces, such as snow, mud, and quicksand. The more pressure you exert on the surface, the more likely it is to deform and flow away, causing you to sink.