Sliding Blocks - Physics Demonstrations - Sliding Blocks | Physics Undergraduate Labs

Purpose

Demonstrates that friction force is independent of contact area. Blocks pulled in series (large contact area) and stacked (small contact area) require the same force on the spring scale, directly challenging the common intuition that "more surface = more friction."

Two wooden blocks arranged in series on a demo bench, connected by string to a large circular 20 N spring scale with red needle, pulled by hand

Same two wooden blocks now stacked on the demo bench, pulled by the same spring scale showing a similar reading

Figure 1: Blocks in series — maximum contact area with the bench surface. The spring scale reads the total friction force.

Figure 2: Blocks stacked — half the contact area, same friction force. The normal force and friction coefficient are unchanged.

Friction Plank

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