Walking Difficulty on Sand vs. Concrete

Walking involves pushing backward against the ground. According to Newton's Third Law, the ground pushes forward on you, enabling movement. The effectiveness of this forward push depends on the surface's ability to resist your backward push.

Surface Properties Comparison

• Concrete Pavement: Concrete is a hard, solid surface. When you step on it, it provides a firm base and exerts a strong forward force (pushback) against your foot. This firm pushback makes walking efficient.
• Sand Surface: Sand consists of loose, granular particles. When you step on sand, the particles beneath your foot shift and move aside (displace).

Reason for Difficulty on Sand

The displacement of sand particles means the surface is not firm and does not provide significant pushback. Instead of pushing forward effectively, much of your energy is spent compacting or displacing the sand. This reduced pushback makes it harder to propel yourself forward, resulting in a feeling of difficulty when walking on sand compared to a solid surface like concrete.

Therefore, the correct explanation is that sand displaces, reducing the pushback needed for efficient walking.