Balanced Forces and Inertia
AdvancedWhen the net force is zero, motion does not change: still things stay still, steady things stay steady. Inertia resists changes of motion — and that is what seatbelts are for.
- Balanced forces cancel, giving zero net force.
- Zero net force: the still stay still and the steady keep moving uniformly.
- Inertia is the resistance to any change of motion.
文體小知識：科普知識文章用生活實例解釋科學概念，先看現象再講原理，讀完就能解釋日常見到的東西。
§Text Passage
1A book lies still on a desk; a high-speed train crosses the fields at 300 km/h — two very different scenes sharing one physical fact: the net force on each is zero. For the book, gravity and the table's support cancel; for the steady train, engine thrust cancels the various resistances.
2That is the key to force and motion: force does not maintain motion, it changes it. With zero net force the stationary stay still and the uniformly moving keep moving; only a non-zero net force alters speed or direction.
3An object's stubborn wish to keep its current state has a formal name: inertia. Lurching forward on a braking bus, dust flying off a shaken sweater — both show inertia, and its size depends on mass: an empty trolley is easy to start and stop, a loaded one is not.
4Inertia also explains road safety: in a crash the car stops but your body keeps going. Seatbelts, headrests and airbags all exist to restrain or cushion that lunge. This closes the forces unit; the next unit enters the living world — classification.
1Summary 重點摘要
When the net force is zero, motion does not change: still things stay still, steady things stay steady. Inertia resists changes of motion — and that is what seatbelts are for.
2Vocabulary cards Vocabulary
Forces acting on one object that are equal in size and opposite in direction, cancelling each other out.
The overall effect of all the forces combined; when it is zero the state of motion stays unchanged.
The upward push that a support such as a table exerts on an object.
The tendency of an object to resist changes in its motion; the greater the mass, the greater the inertia.
The amount of matter in an object and the measure of its inertia, in kilograms.
Motion at an unchanging speed in a straight line; it occurs when the net force is zero.
The restraint that holds the passenger during a crash or sudden braking, countering the forward lunge caused by inertia.
The pulling contest over a rope — a classic example for studying balanced forces and net force.
3Step by step Steps
- Conditions for balanceTwo forces balance only when they are equal in size, opposite in direction, and act on the same object: a book on a table feels downward gravity and the table's upward support, cancelling exactly, so the net force is zero and the book stays still. Remove the table, the support vanishes, the net force is no longer zero and the book falls. Balance is only a cancelling of forces — it never means no forces act.
- Two cases of zero net forceZero net force comes in two cases: a still object stays still, like a book resting on a desk; and a moving object keeps moving in a straight line at constant speed, like a train cruising at 300 km/h, its thrust cancelled by air resistance and friction. In both the state of motion does not change. So deciding whether the net force is zero means looking not just at whether something moves, but whether its speed and direction change.
- Balance in tug of warTug of war is a fine study in net force: with two evenly matched teams, the rope feels pulls equal in size and opposite in direction, cancelling, so it stays motionless; one team pulls a little harder and the net force leans that way, the rope beginning to move. The direction of the net force decides which way the rope moves, its size how fast. To win, besides pulling hard, the grip of your feet on the ground is also decisive.
- What inertia isInertia is an object's resistance to any change of its state of motion: when a bus brakes hard, your feet stop with it but your upper body tips forward, because it tends to keep the original forward motion. Shake a sweater and dust flies off — the sweater moves while the dust, keeping still by inertia, breaks away. Every object has inertia; it is precisely why changing a state of motion requires a force — the heart of Newton's first law.
- Mass and inertiaHow large the inertia is depends on mass: an empty shopping trolley starts and stops easily; a fully loaded one is hard to start and, once moving, hard to stop, because the greater the mass, the greater the inertia and the harder the state of motion is to change. This is why a fully laden lorry needs a much longer braking distance than a car, and why road safety guidance urges large vehicles to slow down early.
- Why seatbelts workSeatbelts are safety design against inertia: in sudden braking or a crash the car stops but the body keeps lunging forward; the belt holds it back, dissolving the lunge before it reaches the windscreen. Headrests stop the head snapping back in a rear collision, and airbags cushion the forward throw — the three work together to protect passengers. Fastening the belt on every journey is both a legal duty and a life-saving habit.
4Diagrams Diagrams
5Worked example Example
Example: a suitcase rests on an airport belt moving at constant speed. Identify the forces on it and explain why its motion does not change.
- The suitcase feels gravity pulling it down.
- The belt pushes up with a normal force equal and opposite to the weight.
- The belt moves steadily and the case does not slide, so the horizontal net force is zero.
- With zero net force the state of motion is unchanged — the case keeps moving with the belt in a straight line.
- Answer: Look at the forces in two directions: vertically, downward gravity and the belt's upward normal force are equal and opposite, balancing each other; horizontally, the case moves at the same speed as the belt with no slipping, so the net force is zero. With zero net force the state of motion is unchanged, so the suitcase keeps moving with the belt in a straight line at constant speed.
6Common mistakes Mistakes
7Quick check Quick check
Q1With zero net force, what happens to a stationary object?
Q2Which two balanced forces act on a book resting on a table?
Q3What is inertia?
Q4Why must you wear a seatbelt in a car?
Q5How are mass and inertia related?
8Recap Recap
- Balanced forces cancel, giving zero net force.
- Zero net force: the still stay still and the steady keep moving uniformly.
- Inertia is the resistance to any change of motion.
- Greater mass means greater inertia.
- Seatbelts counter the forward lunge of inertia — a vital safety device.