Lesson 4: Newton's Laws & Friction | PHY 2.4

Lesson at a Glance | He Tirohanga Whakamua

Do NowWhy a skydiver stops accelerating at terminal velocity10 min

Newton's 3 LawsInertia,

F_{n e t} = m a

, Action-Reaction pairs15 min

Inclined PlanesResolving gravity into

F_{∥} = m g \sin θ

F_{⟂} = m g \cos θ

15 min

Portfolio EntryWrite Section 4: Free-Body Force Diagrams & Inclined Planes10 min

Exit DrillCalculate parallel force down a 20° slope for a 10kg block5 min

Ngā Whāinga Ako | Learning Intentions

Students will know

Newton's 3 Laws of Motion:
1st Law: Inertia ( $Σ F = 0 \Rightarrow a = 0$ ).
2nd Law: $F_{n e t} = m a$ .
3rd Law: Equal & opposite force pairs on separate objects.
How to draw Free-Body Diagrams with scaled force vectors ( $F_{g}, F_{N}, F_{f}, F_{a p p}$ ).
How to resolve gravity ( $m g$ ) on an inclined plane:
• Parallel to slope (causes acceleration down ramp): $F_{∥} = m g \sin θ$
• Perpendicular to slope (balanced by Normal force): $F_{⟂} = m g \cos θ$

Students will demonstrate

By calculating acceleration of a 15kg sled sliding down a 25° hill with friction.
By completing Section 4 of their Level 2 Physics Mechanics Mastery Portfolio.

Do Now | Tīmatanga Whakaaro (10 min)

Terminal Velocity Prompt:

"When a skydiver leaps from a helicopter, gravity pulls them down. Why does their downward speed level off at ~55 m s⁻¹ (terminal velocity) instead of increasing infinitely?"

Unpack: As their falling speed increases, upward air drag ( $F_{d r a g}$ ) increases until it equals downward gravity ( $F_{g} = m g$ ). At this point, net force is zero ( $F_{n e t} = 0$ ). According to Newton's 1st Law, acceleration drops to zero ( $a = 0 {ms}^{- 2}$ ), and the skydiver falls at constant terminal velocity!

Gravity Components on an Inclined Plane (15 min)

1. Down-Slope Force ( $F_{∥}$ )

$F_{∥} = m g \sin θ$
Component of weight pulling the object down the ramp. Opposed by friction ( $F_{f}$ ).

2. Into-Slope Force ( $F_{⟂}$ ) & Normal Force

$F_{⟂} = m g \cos θ$
Component pressing into ramp surface. Equal and opposite to Normal Support Force ( $F_{N} = m g \cos θ$ ).

Newton's Three Laws (15 min)

1. First and second law

First: if $Σ F = 0$ then $a = 0$ , so an object stays still or keeps moving at constant velocity. Constant velocity means balanced forces, not no forces. Second: $F_{n e t} = m a$ , where $F_{n e t}$ is the vector sum. Both laws describe forces acting on ONE object.

2. Third law, and the trap in it

Every force is one half of a pair equal in size and opposite in direction. The pair always acts on TWO DIFFERENT objects, which is why they never cancel. A horse pulls a cart forward; the cart pulls back on the horse. The cart accelerates because the forces on the CART do not balance.

Do this now: for a book resting on a desk, name the third-law partner of the book's weight. It is not the normal force, and saying why is the whole point.

📁 Physics Mechanics Portfolio — Section 4: Free-Body Diagrams & Inclined Planes

Students open their Level 2 Physics Portfolio and complete Section 4:

Section 4 Requirements:

1. Free-Body Force Diagram Pair: Draw scaled free-body diagrams for: 1) Terminal velocity skydiver ( $F_{d r a g} = F_{g}$ ), and 2) Block accelerating down a 30° ramp.

2. Inclined Plane Calculation: Calculate $F_{∥}$ , $F_{⟂}$ , $F_{N}$ , Net Force, and Acceleration for a 20kg crate on a 15° ramp with 25N friction force.

3. Excellence 3rd Law Rationale: 1-paragraph explanation of why the action-reaction force pair of a horse pulling a cart does NOT cancel out ( $F_{h o r s e \to c a r t}$ acts on cart; $F_{c a r t \to h o r s e}$ acts on horse!).

Exit Verification | Ka Mutu Hoki (5 min)

Exit Check:

"My Section 4 correctly calculates down-slope force as mg sin(theta) and explains why Newton's 3rd law action-reaction pairs do not cancel out."

Teacher Planning & NCEA Alignment

NCEA Level 2 Physics Alignment (6 Credits External):

Newton's Laws: Apply $F_{n e t} = m a$ , inertia, and action-reaction pairs to 1D and 2D mechanics scenarios.
Inclined Planes: Resolve gravitational force vectors on inclined surfaces into parallel and perpendicular components.

Vocabulary: Newton's 1st Law, 2nd Law, 3rd Law, free-body diagram, weight ( $m g$ ), normal force ( $F_{N}$ ), friction ( $F_{f}$ ), terminal velocity, inclined plane.

Same concept, shorter route: the Guided Viewing & Problem Practice version of this unit covers it in Newton's Laws and Free-Body Force Diagrams. That route is a compact viewing-and-practice sequence; this one builds the concept over ten sections with a formative portfolio.

Lesson 4: Newton's Laws of Motion & Friction Components