Mechanics, inertia and Newton's first law
Mechanics has two parts, statics and dynamics. Every body resists a change in its state of rest or motion: this is inertia, the idea behind Newton's first law.
Branches of mechanics
| Branch | What it studies | Example |
|---|---|---|
| Statics | Bodies at rest, with forces acting on them | A ceiling fan that is switched off; a book resting on a table |
| Dynamics | Bodies in motion | A bus moving on the road |
| Kinematics (part of dynamics) | Motion without thinking about the force that causes it | Working out how far a cyclist goes in 10 minutes |
| Kinetics (part of dynamics) | Motion together with the force that causes it | Finding the force needed to push a vegetable cart |
Three types of inertia
| Type | Meaning | Everyday example |
|---|---|---|
| Inertia of rest | A body at rest tends to stay at rest | A bus starts suddenly and standing passengers fall backward. Their feet move with the bus, but the upper body stays at rest. |
| Inertia of motion | A moving body tends to keep moving | A moving bus stops suddenly and passengers fall forward. An athlete runs some distance before a long jump. |
| Inertia of direction | A moving body tends to keep moving in the same direction | A bus takes a sharp turn and passengers are pushed sideways, away from the turn. |
Newton's first law
Newton's first law: every body stays in its state of rest or of uniform motion in a straight line. It changes this state only when an external force acts on it. It is also called the law of inertia. The law also tells us what a force is. A force is a push or pull that changes, or tries to change, a body's state of rest or motion. Its SI unit is the newton (N). Contact forces (push, pull, friction, muscular force) act by touching. Non-contact forces (gravitational, magnetic, electrostatic) act from a distance.
Balanced vs unbalanced forces
The resultant is the single force that has the same effect as all the forces together. Forces in the same direction add up (like parallel forces). Forces in opposite directions subtract (unlike parallel forces).
Shake a mango tree and ripe mangoes fall: the branch moves, but the fruit tends to stay at rest (inertia of rest). Seat belts in buses and cars hold you back when the vehicle stops suddenly and your body keeps moving forward (inertia of motion).
1 marks: “Which type of inertia makes passengers fall forward when a bus brakes suddenly?” Inertia of motion: the body keeps moving after the bus stops.
2 marks: “State Newton's first law of motion. Why is it called the law of inertia?” A body stays at rest or in uniform motion in a straight line unless an external force acts on it. It describes inertia, the body's resistance to change.
4 marks: “What is inertia? Explain its three types with one example each.” Define inertia; inertia of rest (bus starts, passengers fall back), of motion (bus stops, passengers fall forward; long jump run-up), of direction (bus turns, passengers pushed sideways); mass measures inertia.
Rest, Motion, Direction: a bus that starts throws you back, a bus that stops throws you forward, a bus that turns throws you sideways.