Understanding Force
FoundationForce is a push or pull, measured in newtons (N). Its three effects: change shape, change speed, change direction. Common forces: gravity, friction and magnetism.
- Force is a push or pull, measured in newtons (N).
- Three effects of force: change shape, change speed, change direction.
- Common forces: gravity, friction, magnetism, tension.
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
1Pushing open the classroom door, unzipping a schoolbag, bending a free kick into the goal — behind every one of these actions lies the same physics concept: force. Force cannot be seen or touched, yet its effects are everywhere.
2Force has three effects: deforming objects (squeezing a sponge), changing speed (a kick starts a ball, a goalkeeper stops it), and changing direction (a racket returns the incoming ball). Force is measured in newtons (N) with a spring balance.
3There are many kinds of force: gravity pulling everything towards the Earth, friction opposing relative motion, the magnetism of magnets, the tension in ropes. The types differ but the effects are the same — changing an object's shape or its state of motion.
4The link between force and motion is the foundation of physics. Over the next two lessons we look first at the most personal force — friction — then learn how graphs and formulas describe motion.
1Summary 重點摘要
Force is a push or pull, measured in newtons (N). Its three effects: change shape, change speed, change direction. Common forces: gravity, friction and magnetism.
2Vocabulary cards Vocabulary
A push or pull between objects that can change shape, speed or direction of motion, measured in newtons (N).
The unit of force, symbol N; a medium apple weighs about 1 N.
The force with which the Earth pulls objects towards it, making them fall.
The size of the gravitational pull on an object; the same object weighs less on the Moon.
The force between two surfaces in contact that opposes relative motion.
The pull a magnet exerts on magnetic materials, or the attraction and repulsion between magnetic poles.
An instrument that measures the size of a force by how far a spring stretches.
A change of shape or size when a force acts, e.g. squeezing a sponge flat.
3Step by step Steps
- Force is push and pullA force is a push or a pull between objects: pushing a door, pulling open a drawer and lifting a schoolbag are all forces at work. Force itself is invisible, but its effects are not — objects move, deform or change direction. Force is measured in newtons (N), and an apple weighs about 1 N. Hitting a ball, carrying water and drawing a bow are forces in daily life. Knowing the definition and unit of force is the first step towards mechanics and physics as a whole.
- Effect 1: changing shapeThe first effect of force is changing shape: press a sponge and it flattens; stretch an elastic band and it lengthens; squeezing plasticine or bending a ruler is deformation too. Some deformations are temporary — release and the object springs back, like elastic bands and springs; others are permanent, like squashed plasticine. Engineering must account for deformation: bridge cables bear tension while seat foam cushions pressure by deforming.
- Effect 2: changing speedThe second effect of force is changing speed: kick a still football and it starts moving and speeds up; the goalkeeper catches it and it turns from fast to stopped. Force can start an object moving, speed it up or slow it to a halt. Changes of how fast, and of direction, both count as changes of the state of motion. Braking, setting off and throwing in daily life are all forces changing the speed of objects.
- Effect 3: changing directionThe third effect of force is changing the direction of motion: a racket sends the incoming tennis ball straight back, reversing its direction; a curved free kick bends through the air because the force is applied off-centre. A change of direction is also a change of the state of motion — even at constant speed, if the direction changes a force has acted. A satellite circling the Earth changes direction every moment, exactly the result of gravity acting continuously.
- Types of forceForces come in different types: a falling apple feels gravity; your shoe meets friction on the ground; a magnet attracts paper clips magnetically; a rope holds a weight by tension; a desk supports a book with a normal push. Many types, yet all produce pushes, pulls and deformation, and all share the unit of the newton. In analysis, first identify which forces act on an object and from which directions, then study the effects — the basic habit of mechanics.
- Measuring forceForce is measured with a spring balance: hang a 500 g object beneath it and the reading is about 5 N — the heavier the object, the larger the reading. The principle is that a spring stretches under a pulling force, with extension proportional to the force, so a pointer marks the value. Set the pointer to zero before use for accurate readings, and hang the object vertically to avoid friction. This way of measuring force through deformation is still widely used today.
4Diagrams Diagrams
5Worked example Example
Example: a footballer kicks the ball, and the goalkeeper later catches it. Identify the forces and their effects.
- The kick: force starts the still ball moving and speeds it up — changing speed.
- In flight gravity bends the path gradually downwards — changing direction.
- The catch: force stops the moving ball — changing speed again.
- Pushing off, the player's boots grip the grass by friction — friction at work.
- Answer: When the player kicks, the force starts the still ball moving and speeds it up — changing speed; in flight gravity bends the path gradually towards the ground — changing direction; when the goalkeeper catches it, the force stops the moving ball — changing speed again. Besides, friction between boots and grass helps the player push off. The whole sequence shows several effects of force.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is the unit of force?
Q2State the three effects of a force.
Q3Which force acts on an apple falling from a tree?
Q4Which instrument measures the size of a force?
Q5What happens when two north poles of magnets come close?
8Recap Recap
- Force is a push or pull, measured in newtons (N).
- Three effects of force: change shape, change speed, change direction.
- Common forces: gravity, friction, magnetism, tension.
- Measure force with a spring balance.
- Stationary objects still feel forces — they are simply balanced.