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

Biomechanics — Force, Momentum and Impulse

Biomechanics explains why technique works. Newton's laws, momentum and impulse tell you how to hit harder, land softer and change direction faster.

Learning objective: Apply Newton's laws, momentum and impulse to improve movement.

By the end of this lesson you can:

• I can apply Newton's three laws to sport.
• I can calculate momentum and impulse.
• I can explain how to increase or reduce impulse.

Momentum is a bowling ball. A heavy, fast bowling ball knocks over more pins than a light, slow one. Momentum (mass × velocity) explains why heavier, faster players are harder to stop.

Biomechanics — Force, Momentum and Impulse

NEWTON S LAWS:
• FIRST (inertia) — a body stays at rest or constant velocity unless a force acts.
• SECOND (acceleration) — F = m × a.
• THIRD (action–reaction) — every action has an equal and opposite reaction.

MOMENTUM:
• Momentum = mass × velocity (kg m/s).
• Conservation of momentum in collisions; summation of momentum through the body.

IMPULSE:
• Impulse = force × time = change in momentum.
• To INCREASE momentum (throwing, hitting): apply force over a longer time (follow-through, longer backswing).
• To REDUCE impact force (landing, catching): increase the time of impact (bend knees, "give" with the hands).

| Concept | Formula | Application |
|---|---|---|
| Momentum | p = m × v | Tackling, collisions |
| Impulse | I = F × t = Δp | Follow-through, landing |
| Newton's 1st | Inertia | Body stays at rest until force applied |
| Newton's 2nd | F = m × a | More force → more acceleration |
| Newton's 3rd | Action–reaction | Pushing off blocks |
| Conservation of momentum | Total before = total after | Summation of momentum in throws |

Key ideas at a glance

• Show formulas and units: kg·m/s, N·s.
• Increase time to absorb force: Bend knees on landing.
• Increase time to generate impulse: Long follow-through.
• Link to Newton's laws: Name the law.

Key formulas

• Momentum: p = mv
• Impulse: I = F \times t = \Delta p
• Newton's second law: F = ma

Worked example (Medium)

A 90 kg rugby player runs at 7 m/s. Calculate momentum.

1. Formula. p = mv
2. Calculate. 90 × 7 = 630 kg m/s

Answer: 630 kg m/s

Worked example (Medium)

A 90 kg rugby player runs at 7 m/s. Calculate momentum.

1. Formula. p = m × v.
2. Substitute. 90 × 7.
3. Answer. 630 kg·m/s.

Answer: 630 kg·m/s.

Worked example (Hard)

Explain using impulse why a goalkeeper "gives" with the ball.

1. Concept. Impulse = F × t.
2. Action. Moving hands back increases time of contact.
3. Effect. For the same change in momentum, force is reduced.

Answer: Increasing contact time reduces force on the hands.

Common mistakes

• Saying bending the knees reduces impulse Impulse is the same; force is reduced because time increases. Fix: Explain the force–time relationship.
• Forgetting units Momentum is kg m/s. Fix: Always include units.
• Forgetting momentum has direction It's a vector. Fix: Include direction when relevant.

Impulse–time graphs

On a force–time graph, impulse is the area under the curve.

A sprinter at the blocks wants a large area quickly (high force over enough time). A landing gymnast wants a long, low curve (force spread over more time) to reduce injury risk.

Practice questions

1. Bending the knees when landing reduces force by:

   1. Reducing mass
   2. Increasing the time of impact
   3. Reducing impulse
   4. Increasing velocity

   (1 marks)

2. A 0.45 kg ball leaves the foot at 20 m/s from rest. Find its momentum and the impulse applied.

   (2 marks)

3. Momentum equals:

   1. F × t
   2. m × v
   3. m × a
   4. v ÷ t

   (1 marks)

4. Bending knees on landing reduces force by:

   1. Reducing mass
   2. Increasing time of impact
   3. Increasing velocity
   4. Newton's 1st law

   (1 marks)

5. Calculate impulse if a force of 600 N acts for 0.2 s.

   (2 marks)

Summary

• Newton's three laws explain motion.
• Momentum p = mv.
• Impulse = F × t; manipulate time to change force.
• p = mv; I = Ft; increase time to reduce or generate force.