VCE Physical Education (Year 12) · Unit 3 — Movement Skills · 16m read

Biomechanics — Levers, Projectiles and Angular Motion

Why do golfers use long clubs and divers tuck tight? Levers, projectiles and angular motion explain it — and they appear in almost every Unit 3 exam.

Learning objective: Analyse levers, projectile motion and angular motion in sporting skills.

By the end of this lesson you can:

• I can identify lever classes and mechanical advantage.
• I can explain factors affecting projectile distance.
• I can apply angular momentum to rotations.

Levers are the body's toolkit. A crowbar lets you lift heavy things; a fishing rod lets you flick a lure far and fast. Body levers trade force for speed — most give speed and range at the cost of force.

Biomechanics — Levers, Projectiles and Angular Motion

LEVERS:
• Parts: axis (fulcrum), force (effort), resistance (load).
• FIRST CLASS — axis in the middle (neck nodding).
• SECOND CLASS — resistance in the middle (calf raise).
• THIRD CLASS — force in the middle (biceps curl); most body levers; favour speed and range of movement.
• Longer levers produce greater velocity at the end (implements like bats and racquets).

PROJECTILE MOTION:
• Factors: SPEED of release (most important), ANGLE of release, HEIGHT of release.
• If release and landing heights are equal, the optimal angle is 45°; if release is higher than landing, the optimal angle is less than 45°.
• Air resistance and spin (Magnus effect) also matter.

ANGULAR MOTION:
• Angular momentum = moment of inertia × angular velocity.
• Moment of inertia decreases when mass is brought closer to the axis (tuck) → spins faster.
• Angular momentum is conserved in the air.

| Lever class | Order (middle) | Body example | Advantage |
|---|---|---|---|
| 1st class | Axis in middle (F–A–R) | Neck nodding | Balance |
| 2nd class | Resistance in middle (A–R–F) | Calf raise | Force |
| 3rd class | Force in middle (A–F–R) | Elbow flexion | Speed and range |

Key ideas at a glance

• Longer lever = more end speed: Racquets, straight arms.
• Projectile factors: Speed, angle, height of release.
• Optimal angle ~45° (equal heights): Lower if release above landing.
• Angular momentum conserved in flight: Tuck to spin faster.

Worked example (Medium)

Explain why a diver tucks to complete more somersaults.

1. Conservation. Angular momentum stays constant in the air.
2. Tuck. Reduces moment of inertia.
3. Result. Angular velocity increases — faster rotation.

Answer: Tucking lowers moment of inertia, so angular velocity rises to conserve angular momentum.

Worked example (Hard)

Explain why a shot putter releases at about 37–40°, not 45°.

1. Release height. Released above landing height.
2. Effect. Optimal angle drops below 45°.
3. Also. Athletes generate more speed at lower angles.

Answer: Release height above landing and greater release speed at lower angles make <45° optimal.

Common mistakes

• Saying 45° is always optimal Only true when release and landing heights are equal. Fix: Consider release height.
• Ignoring release speed Speed has the biggest effect on distance. Fix: Rank speed first.
• Saying 45° is always optimal Only when release and landing heights are equal. Fix: Account for release height.

Angular momentum

Angular momentum = moment of inertia × angular velocity. In the air it is conserved.

A diver who tucks reduces moment of inertia, so angular velocity increases — they spin faster. Opening out slows rotation for a clean entry.

Practice questions

1. A biceps curl is which class of lever?

   1. First
   2. Second
   3. Third
   4. Fourth

   (1 marks)

2. Why might a shot putter release at about 38–40° rather than 45°?

   (2 marks)

3. Most body levers are:

   1. 1st class
   2. 2nd class
   3. 3rd class
   4. None

   (1 marks)

4. Tucking during a somersault:

   1. Slows rotation
   2. Speeds rotation
   3. Has no effect
   4. Increases moment of inertia

   (1 marks)

5. List three factors affecting projectile distance.

   (3 marks)

Summary

• Third-class levers favour speed and range.
• Projectile distance: speed, angle, height.
• Reduce moment of inertia to spin faster.
• Levers trade force for speed; projectiles depend on speed, angle, height.