Topics Covered
10 key topics in this chapter
Study Resources
Key Concepts
01. Balanced & Unbalanced Forces
Balanced forces produce no change in state (net force = 0). Unbalanced forces change the state of rest or motion. An object continues in its current state unless acted on by an unbalanced force.
02. Newton's First Law — Inertia
Every body continues in its state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force. Inertia is the tendency to resist change; it is proportional to mass.
03. Newton's Second Law — F = ma
The rate of change of momentum of a body is directly proportional to the applied unbalanced force and takes place in the direction of the force. F = ma. SI unit of force: Newton (N = kg·m/s²).
04. Momentum
Momentum p = mv (mass × velocity). It is a vector (same direction as v). SI unit: kg·m/s. A large truck at low speed can have the same momentum as a small car at high speed — both are equally hard to stop.
05. Newton's Third Law
For every action there is an equal and opposite reaction. Forces always occur in pairs — action and reaction act on different bodies simultaneously. Example: rocket propulsion, swimming, recoil of a gun.
06. Conservation of Momentum
In the absence of an external force, the total momentum of a system remains constant. m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂. Derived from Newton's third law — action-reaction impulses cancel.
Formulas at a Glance
| # | Name | Expression | Notes |
|---|---|---|---|
| 01 | Momentum | p = mv |
mass × velocity (kg·m/s) |
| 02 | Newton's 2nd Law | F = ma = Δp/Δt |
Force = rate of change of momentum |
| 03 | 1 Newton | 1 N = 1 kg·m/s² |
Force to accelerate 1 kg at 1 m/s² |
| 04 | Impulse | J = F×t = Δp |
Impulse = change in momentum |
| 05 | Conservation of p | m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂ |
No external force acts on system |
| 06 | Recoil Velocity | v_gun = −(m_bullet × v_bullet)/m_gun |
From conservation of momentum |
Important Notes
Exam Tips & Common Mistakes
State Newton's laws formally before explaining the application — examiners deduct marks for skipping the statement.
In conservation of momentum problems: if one object is initially at rest (u₂ = 0), simplify m₁u₁ = m₁v₁ + m₂v₂.
"A man jumps out of a boat" — boat recoils backward. Apply: total initial momentum = 0, so mv_man + mv_boat = 0.
Impulse = F × t = Δp — used to explain why catching a cricket ball slowly hurts less (longer time → smaller force).
Distinguish mass (inertia, scalar, constant) from weight (gravitational force, vector, varies with g).