Arm Model - Physics Demonstrations - Arm Model | Physics Undergraduate Labs

Purpose

A mechanical arm model demonstrates the human arm as a lever system. Students measure the muscle force required to hold a load and see how it depends on arm angle and attachment point, illustrating torque equilibrium and mechanical advantage in a biological context.

Anatomical arm model with painted muscles mounted upright on a black base at the edge of a lecture bench, with a cord running up to a large circular spring scale (0-20 N) mounted above. The forearm is bent at roughly 90 degrees and holds a blue ball.

Same arm model with the forearm extended to a more open angle. The spring scale needle has moved to a higher reading, showing increased muscle force required at this arm angle.

Side view of the arm model with the forearm nearly horizontal, holding the blue ball at full extension. A larger spring scale (0–40 N range) is now used, with the needle reading around 18 N. A hand is visible adjusting the cord.

Figure 1: Arm model configured for bicep action with the 0–20 N spring scale. The cord simulates the bicep muscle; the scale reads the tension required to hold the blue ball.

Figure 2: With the forearm extended further, the spring scale shows increased muscle force — the changing moment arm requires more tension to maintain equilibrium.

Figure 3: Arm nearly horizontal holding the ball at full extension, now using a 0–40 N scale. This position maximizes the load moment arm, requiring substantially greater muscle force (~18 N) to maintain equilibrium.