Dynamics
Covers types of forces, mass, weight, gravitational field, Newton’s Laws of Motion, and effects of resistive forces on motion.
Summary Table
| Topic | Definition | Formula | Unit | Notes |
|---|---|---|---|---|
| Forces | Push or pull acting on an object; can be contact or non-contact. | - | N (Newton) | Contact forces: friction, tension, normal reaction, upthrust; Non-contact: gravitational, electrostatic, magnetic. |
| Mass | Measure of the amount of matter; resists changes in motion (inertia). | - | kg | Different from weight; does not change with location. |
| Weight | Force due to gravity acting on a mass. | Weight = Mass × Gravitational field strength (W = m × g) | N | g ≈ 9.81 N/kg on Earth; varies with location. |
| Gravitational Field Strength | Force per unit mass in a gravitational field. | g = W / m | N/kg | Also called gravitational acceleration. |
| Newton's 1st Law | A body remains at rest or in uniform motion unless acted on by a resultant force. | - | - | Also known as the law of inertia. |
| Newton's 2nd Law | Resultant force causes acceleration proportional to mass. | F = m × a | N = kg·m/s² | Use consistent units when solving problems. |
| Newton's 3rd Law | For every action, there is an equal and opposite reaction. | - | - | Action and reaction forces act on different bodies. |
| Friction | Resistive force opposing relative motion between surfaces. | F_friction = μ × N | N | μ = coefficient of friction; N = normal force. |
| Air Resistance | Resistive force opposing motion through air. | - | N | Increases with speed; causes falling objects to reach terminal velocity where air resistance = weight and net force = 0 N. |
| Falling Bodies | Motion under gravity; can be with or without air resistance. | v = u + at; s = ut + 1/2 at²; v² = u² + 2as | m/s² for a, m for s, m/s for v & u | Without air resistance: a = g; With air resistance: acceleration decreases until terminal velocity. |
- ⚠️ Mass and weight are the same thing.
- ⚠️ An object moving at constant speed must have a resultant force acting on it.
- ⚠️ Heavier objects fall faster in free fall.
- ⚠️ Action and reaction forces act on the same body.
- 👉 Always label forces clearly in free-body diagrams with both magnitude and direction.
- 👉 Differentiate between mass (kg) and weight (N) in explanations and calculations.
- 👉 Remember that Newton’s 3rd law pairs act on different objects, not the same one.
- 👉 When solving F = ma, ensure units are consistent (N, kg, m/s²).
- 👉 For terminal velocity, explain the balance between weight and air resistance clearly.