Third Class Levers in the Human Arm

This activity demonstrates how the human forearm works as a third-class lever. Students test lifting a bucket of sand with their arm and then create a cardboard model to visualize how muscles act as input forces.

1. Fill a bucket halfway with sand and place your forearm flat on a table, palm up, with your hand extending past the edge.
2. Have a helper hang the bucket from your palm and lift by bending your elbow.
3. Add more sand and repeat, observing how the effort changes.
4. Place a PVC pipe or yardstick in your hand and hang the bucket from its far end. Try lifting again and compare the effort.
5. Move the bucket along the stick to different positions and note changes in effort.
6. Make a cardboard model of the arm: cut out upper arm, forearm, and hand pieces.
7. Join upper arm and forearm with a brad to act as the elbow fulcrum.
8. Attach paperclips to represent the bicep attachment points. Thread string through them to model the muscle.
9. Pull the string to simulate bicep contraction and observe how the hand moves.
10. Add a small weight to the hand model and test again.

📄 Third-Class Levers in the Human Body - Education.com: https://www.education.com/activity/article/human-machine/

• Build a tricep muscle model by adding extra paperclips and string to straighten the arm.
• Use different weights to test the effect of load size.
• Compare the human arm lever with everyday third-class levers like baseball bats, tweezers, or shovels.

• Ensure the bucket is not too heavy to avoid muscle strain.
• Use caution when cutting cardboard with scissors.
• Supervise younger students during construction and lifting activities.

• Why does lifting the bucket become harder when you add more sand? (Because the weight increases, requiring more input force.)
• Why does lifting feel harder when the bucket hangs farther out on the stick? (Because the output arm is longer, increasing the force required.)
• In your cardboard model, why does the hand move farther than the string is pulled? (Because in third-class levers, the output moves a greater distance than the input.)
• What advantages does the human arm have as a third-class lever? (It allows fast, wide movements even though it requires more effort.)