Exploring Forces and Motion
FoundationForces are invisible, yet they change an object's shape, speed and direction. Common forces are push, pull, gravity, friction, magnetic force and elastic force. Measure with a spring balance and change only one variable at a time in a fair test, and you can find how friction depends on how rough a surface is.
- Force is invisible yet real — it changes an object's shape, speed and direction, measured in newtons.
- Common forces include push, pull, gravity, magnetic force, elastic force and friction.
- Friction grows with rougher surfaces and tighter pressing; sole patterns add it, oil and wheels reduce it.
文體小知識：科學探究實驗報告記錄提出問題、假設、實驗和結論的過程，教人用公平測試尋找答案。
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1Forces cannot be seen directly, but we notice their effects: squeeze plasticine and it changes shape — a force can change an object's shape; kicking a scooter makes it move and braking slows it — a force can change speed; a ping-pong ball curves after a stroke — a force can change direction. Force is measured in newtons with a spring balance. Common forces include pushes and pulls, gravity that pulls objects towards the ground, magnetic force that attracts iron from afar, the elastic force of springs and rubber bands, and friction between any two surfaces in contact.
2Friction opposes relative motion between surfaces. Its size depends on how rough the surfaces are and how hard they press together: the rougher and tighter, the greater the friction. Friction helps and hinders. Shoe soles and tyre treads add grip so we walk and drive safely; oiling a bicycle chain and adding bearings to machines reduce friction for smooth running. A box on small wheels is far easier to move than one dragged along the ground, because rolling friction is much less than sliding friction.
3Scientists explore forces and motion through fair tests. First ask a question — for example, 'does a rougher surface give more friction?' — then propose a hypothesis and design the experiment. The key is controlling variables: change only the one condition under study and keep everything else the same. To compare friction on different surfaces, use the same block and the same spring balance, pull at a steady speed, and change only the material under the block; measure three times for each surface and average to reduce chance error, then draw a conclusion from the data.
4Knowledge of forces and motion serves us everywhere. Seat belts and air bags protect passengers by cushioning crashes; a helmet's shell and lining absorb impact; ramps make pushing loads upstairs easier; maglev trains use magnetic force to float above the track, escaping most friction to travel at high speed. Understanding the rules of force lets us design safer, easier tools and structures.
1Summary 重點摘要
Forces are invisible, yet they change an object's shape, speed and direction. Common forces are push, pull, gravity, friction, magnetic force and elastic force. Measure with a spring balance and change only one variable at a time in a fair test, and you can find how friction depends on how rough a surface is.
2Vocabulary cards Vocabulary
An action that changes an object's shape or motion, measured in newtons.
The Earth's pull on objects, directed downwards, making things fall.
The force between two touching surfaces that opposes their relative motion.
The force a magnet exerts on magnetic materials, acting across space.
The restoring force produced when springs or rubber bands are stretched or squeezed.
A tool that measures force by how far a spring stretches, in newtons.
The unit of force; an object of about 100 grams feels a gravity of about one newton.
A test in which only the variable being studied changes and all other conditions stay the same.
A condition that can change in an experiment; a fair test changes only one at a time.
3Step by step Steps
- Step 1: Know force by its effectsExample: squeeze plasticine hard and it changes shape — force can change the shape of an object; push off on a scooter and it moves from rest — force can change an object's state of motion; a ping-pong stroke sends the ball back the other way — force can change the direction of motion. Although force itself is invisible, we can tell it is there from the changes it makes to shape, speed and direction.
- Step 2: Feel gravity and measure itExample: release an apple and it falls straight down to the ground — the result of gravity, whose direction points to the centre of the Earth. Hang an object of about one hundred grams on a spring balance and it reads about one newton, the newton being the unit of force. Gravity makes raindrops fall, brings a jumping person back to the ground, and lets us stand firmly on the Earth. Without gravity, water could not be poured and the whole world would be in chaos.
- Step 3: Compare friction on different surfacesExample: walking on ice is easy to slip, while walking on a rubber mat is steady, because smooth ice has little friction and a rough mat has much. The patterns on shoe soles are designed to add friction and prevent slipping; but the rougher the surface, the more effort is needed to drag an object across it. The amount of friction depends on what the surfaces are made of and how rough they are — the very point our next experiment will test.
- Step 4: Weigh friction's good and bad sidesExample: an oiled bicycle chain is easier to pedal — reducing harmful friction; yet the oil must never touch the tyres, or there will not be enough friction to brake and the bicycle cannot stop, which is very dangerous. So friction sometimes needs reducing and sometimes increasing: walking, holding a pen and braking all rely on friction's help. The standard for judging is whether the friction helps us or gets in our way.
- Step 5: Roll instead of slideExample: pushing a heavy box fitted with small wheels is far easier than dragging it along the ground, because rolling friction is much less than sliding friction. The wheels under a luggage case, shopping trolleys and the casters of office chairs all use this principle. Ancient people moving huge stones also laid round logs beneath them so the stones could 'roll' forward. Whenever something heavy must be moved, first think whether rolling can replace sliding.
- Step 6: Design a fair testExample: to study how surface roughness affects friction, drag the same block with the same spring balance across a desk, a towel and sandpaper, pulling at a steady speed and recording each reading. The experiment changes only one condition — the surface — and keeps everything else the same; this is what makes it a fair test. If the block or the pulling speed changed at the same time, we could not tell which factor caused the reading to change.
- Step 7: Repeat, measure, let data speakExample: measure each surface three times and take the average to reduce accidental error — if the reading is largest on sandpaper, middle on the towel and smallest on the desk, we can conclude that the rougher the contact surface, the greater the friction. Conclusions must be supported by data instead of guessed by feeling; even when results differ from expectations, we record them honestly and check again — the honest attitude of scientific inquiry.
4Diagrams Diagrams
5Worked example Example
Example: Hon wants to test whether an object's weight affects friction. He has one wooden box, one spring balance and one smooth board, and will add different numbers of weights inside the box. Design a fair test for him, write the steps and predict the result.
- Hypothesis: the heavier the box, the tighter it presses the surfaces, and the greater the friction.
- Control the variables: use the same box, the same board and the same spring balance throughout, pulled at a steady speed; change only the number of weights inside.
- Steps: measure the pull with the empty box, then with one, two and three weights added, measuring three times each and averaging.
- Record: suppose the empty box reads about 0.5 newtons; with one weight about 0.8, with two about 1.1, and with three about 1.4.
- Conclusion: on the same surface, the greater the weight, the greater the friction — matching the hypothesis.
- Answer: Using the same box and board, changing only the weight, and repeating measurements to average is a fair test; the results show that with the same surface, more weight brings more friction — the tighter the press, the greater the friction.
6Common mistakes Mistakes
7Quick check Quick check
Q1What three effects can a force have on an object?
Q2What does the size of friction depend on?
Q3Why do shoe soles have patterns?
Q4What is a fair test?
Q5Name two everyday ways of reducing friction.
8Recap Recap
- Force is invisible yet real — it changes an object's shape, speed and direction, measured in newtons.
- Common forces include push, pull, gravity, magnetic force, elastic force and friction.
- Friction grows with rougher surfaces and tighter pressing; sole patterns add it, oil and wheels reduce it.
- A fair test changes one variable at a time, keeps the rest constant, and repeats measurements to average.
- Knowledge of forces and motion shapes seat belts, helmets, ramps and maglev trains.