All chapters / 3

Class 9–10 · Physics

Force

Change it and see

Force

F₁F₂Fnet = F₂ − F₁
Opposite forces on one object combine as a vector difference.
Lessons in this chapter
  1. Forces and free-body diagrams
  2. First law and inertia
  3. Second law and momentum
  4. Third law
  5. Impulse
  6. Momentum conservation
  7. Friction
  8. Gravitation, mass and weight
  9. Torque and equilibrium

Lessons

1

Forces and free-body diagrams

F₁F₂Fnet = F₂ − F₁
Opposite forces on one object combine as a vector difference.

Forces can change motion or shape. A free-body diagram includes only forces acting on the selected object. Add them as vectors. Zero net force permits rest or constant straight-line velocity.

Key relationship

Fnet = vector sum of all forces

Worked example

8 N right − 3 N left = 5 N right

2

First law and inertia

F₁F₂Fnet = F₂ − F₁
Opposite forces on one object combine as a vector difference.

Without a net external force, velocity remains constant. Inertia is resistance to a change of motion, measured by mass. When a bus stops, your body tends to continue; a seat belt supplies a stopping force.

Key relationship

Fnet = 0 ⇒ velocity constant

Worked example

Same force: 2 kg accelerates more than 4 kg.

3

Second law and momentum

F₁F₂Fnet = F₂ − F₁
Opposite forces on one object combine as a vector difference.

Momentum is mass times velocity. Net force equals the rate of change of momentum. For constant mass, this becomes F = ma. Use the resultant of all forces rather than a single applied force.

Key relationship

p = mv; F = Δp/Δt; Fnet = ma

Worked example

m = 4 kg; applied force 12 N, friction 4 N ⇒ a = (12−4)/4 = 2 m/s²

4

Third law

F₁F₂Fnet = F₂ − F₁
Opposite forces on one object combine as a vector difference.

Interaction forces are equal and opposite and act simultaneously on different objects. A foot pushes the ground backward and the ground pushes the foot forward. They do not cancel in a diagram of only one object.

Key relationship

FA→B = −FB→A

Worked example

Book pushes table downward; table pushes book upward.

5

Impulse

F₁F₂Fnet = F₂ − F₁
Opposite forces on one object combine as a vector difference.

Impulse equals change in momentum. Spreading the same change over a longer time reduces average force. Moving hands backward while catching a ball and vehicle crumple zones both extend stopping time.

Key relationship

J = Favg Δt = Δp

Worked example

0.2 kg ball: 10→0 m/s ⇒ magnitude Δp = 2 kg m/s; in 0.1 s ⇒ 20 N

6

Momentum conservation

F₁F₂Fnet = F₂ − F₁
Opposite forces on one object combine as a vector difference.

Total momentum is conserved when net external force is zero. Keep one sign convention before and after impact. Objects that stick share a final velocity. Kinetic energy is additionally conserved only in an elastic collision.

Key relationship

m₁u₁+m₂u₂ = m₁v₁+m₂v₂

Worked example

2 kg at 3 m/s sticks to stationary 1 kg ⇒ 6 = 3v ⇒ v = 2 m/s

7

Friction

F₁F₂Fnet = F₂ − F₁
Opposite forces on one object combine as a vector difference.

Friction opposes relative sliding or its tendency at a contact. Static friction adjusts up to a limiting value; kinetic friction acts during sliding. Grip enables walking, while lubrication reduces unwanted resistance in machinery.

Key relationship

fs ≤ μsN; limiting fs = μsN; fk ≈ μkN

Worked example

N = 50 N, μs = 0.4 ⇒ limit 20 N; stationary under 10 N push ⇒ friction 10 N

8

Gravitation, mass and weight

F₁F₂Fnet = F₂ − F₁
Opposite forces on one object combine as a vector difference.

All masses attract. Weight is gravitational force, whereas mass measures inertia. Changing g changes weight but not mass. Astronauts feel weightless because they and their vehicle fall together, not because gravity is absent.

Key relationship

F = Gm₁m₂/r²; W = mg

Worked example

m = 50 kg ⇒ Earth weight 490 N; at g = 1.6 m/s² ⇒ 80 N

9

Torque and equilibrium

F₁F₂Fnet = F₂ − F₁
Opposite forces on one object combine as a vector difference.

Torque equals force times perpendicular lever arm. Rotational equilibrium requires zero net torque; complete static equilibrium also requires zero net force. An object may topple when the vertical through its centre of gravity leaves its support base.

Key relationship

τ = Fd⊥; ΣF = 0; Στ = 0

Worked example

20 N × 0.3 m = 10 N × 0.6 m = 6 N m

Check your understanding

Try answering first, then open the question to compare your reasoning.

1. Net force 20 N on 5 kg: acceleration?

a = 20/5 = 4 m/s²

2. Do action–reaction forces act on one object?

No: A acts on B, B acts on A

3. Same Δp, twice the time: average force?

F = Δp/Δt ⇒ half

4. When is total momentum conserved?

Net external force = 0

5. If g halves, what happens to mass and weight?

Mass unchanged; W = mg ⇒ weight halves

Sources and scope

Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.