Question

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

• A. Sand is soft and concrete is hard
• B. Friction between sand and feet is less than on concrete
• C. Friction between sand and feet is more than on concrete
• D. Sand is grainy and concrete is smooth

Hint:

Walking forward needs a firm grip from friction against the ground beneath the foot.

Answer 1

The Correct Option is B

Step 1: Walking forward needs friction as a push-off force. When a foot pushes backward against the ground, the ground pushes forward on the foot in return, by Newton's third law, and this forward push comes from friction between the foot and the surface.

Step 2: On sand, the loose grains shift and slide under the foot with every step. Because the surface itself gives way, much of the muscular effort put into each step goes into displacing the sand rather than generating a firm forward push.

Step 3: Concrete is rigid and does not give way. It offers a firm, stable surface, so nearly all of the backward push from the foot is converted into an equal firm forward push, using the available friction efficiently.

Step 4: Because the shifting sand offers a lower effective grip for the forward push compared to the firm grip on concrete, walking on sand takes more effort. This is exactly what a lower effective friction between sand and feet, compared to concrete, means.

Answer: Friction between sand and feet is less than on concrete, option B.