Introduction
Newton's Laws of Motion are the engine of classical mechanics. In 9709 Mechanics, the most frequently examined skill is applying Newton's Second Law () to find the acceleration of, or the forces acting on, a particle moving in a straight line. Questions routinely feature inclined planes, connected particles joined by strings, friction, and resisting forces. Mastery of this topic is essential: it underpins every dynamics question in the paper.
Core Concept
Newton's Three Laws (as applied in 9709)
- First Law: A particle remains at rest or moves with constant velocity unless acted upon by a resultant force.
- Second Law: The resultant force on a particle equals the product of its mass and acceleration: .
- Third Law: If particle A exerts a force on particle B, then B exerts an equal and opposite force on A.
Setting Up an Equation of Motion
The strategy is always the same:
- Draw a force diagram — mark every force acting on the particle.
- Choose a positive direction (usually the direction of motion or of acceleration).
- Apply , taking forces in the positive direction as positive and those opposing it as negative.
- Solve for the unknown.
Key Force Types
| Force | Symbol | Notes |
|---|---|---|
| Weight | Always acts vertically downward | |
| Normal reaction | or | Perpendicular to the surface; found from equilibrium perpendicular to motion |
| Friction | Opposes motion; at limiting friction | |
| Tension | Acts along a string, away from the particle | |
| Thrust | Acts along a connecting rod, towards the particle (compressive) | |
| Air resistance / driving force | Given in question | Direction stated or deducible |
Normal Reaction on an Incline
For a particle on a plane inclined at angle to the horizontal, resolving perpendicular to the plane (no acceleration in that direction):
The component of weight along the plane (down the slope) is .
Unlock the full Newton's Laws of Motion note with Nova
You're reading the preview. Unlock the complete note — every worked example, examiner pitfall and practice question — plus 24/7 AI tutoring from Nova that teaches directly from these notes.