It is more difficult to walk on sand than on concrete because:

Question

A body of mass 2 kg is moving with a velocity of 3 m/s. What is its kinetic energy?

Answer 1

Think about it in terms of where the effort of each step actually goes.

Walking on any surface needs friction to convert the backward push of the foot into forward motion of the body.
Sand grains shift and settle under the foot with each step, so part of the energy spent in that step is used up moving and compacting the sand rather than pushing the body forward.
Concrete does not deform at all under a footstep, so it offers firm, dependable grip, and nearly all of the pushing effort goes straight into forward motion.
Even though sand feels rough and grainy, the reliable forward grip it offers to a foot is actually less than the grip offered by rigid concrete, since the loose sand keeps shifting instead of holding firm.

This lower effective friction between the foot and the shifting sand is why walking on sand is harder, so option B is correct.

Answer 2

Think about it in terms of where the effort of each step actually goes.

Walking on any surface needs friction to convert the backward push of the foot into forward motion of the body.
Sand grains shift and settle under the foot with each step, so part of the energy spent in that step is used up moving and compacting the sand rather than pushing the body forward.
Concrete does not deform at all under a footstep, so it offers firm, dependable grip, and nearly all of the pushing effort goes straight into forward motion.
Even though sand feels rough and grainy, the reliable forward grip it offers to a foot is actually less than the grip offered by rigid concrete, since the loose sand keeps shifting instead of holding firm.

This lower effective friction between the foot and the shifting sand is why walking on sand is harder, so option B is correct.

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