Torque Bar - Physics Demonstrations - Torque Bar | Physics Undergraduate Labs

Clear the area beneath the demonstration to prevent foot injuries if the volunteer loses control and the bar rotates rapidly, potentially releasing the mass. Have volunteers wear closed-toe shoes and keep other students at a safe distance.

Purpose

A volunteer holds a bar by its handle and tries to keep it horizontal while a 1 kg mass is hung from progressively farther notches. Students feel (or watch someone feel) that torque increases linearly with lever arm distance, even though the force stays constant.

Hand gripping wooden cylindrical handle with thin metal rod extending left; a brass 1 kg mass hangs from a notch near the middle of the rod

Two hands gripping the wooden handle as the brass mass hangs from a notch near the far end of the rod, demonstrating the increased difficulty

Torque bar apparatus on green surface: wooden cylindrical handle perpendicular to a thin metal rod with six equally-spaced notches, brass 1 kg hooked mass beside it

Figure 1: The 1 kg mass hung from a notch near the handle — manageable torque at this short lever arm.

Figure 2: With the mass at a far notch, the increased lever arm makes it much harder to keep the bar horizontal.

Figure 3: Complete apparatus: wooden handle, metal rod with six notches, and 1 kg brass hooked mass.

Safety Warnings

Warning: Dropped Mass Precaution

Purpose