Exploring Forces
Why is it hard to pedal uphill but easy downhill? Why do you slip on ice? It's all about forces — the pushes and pulls all around you.
What is a force?
A force is a push or pull on an object, resulting from that object's interaction with another object. The SI unit of force is the newton, symbol N.
| A force applied on an object CAN… |
|---|
| Make it start moving from rest |
| Change its speed if it's already moving |
| Change its direction of motion |
| Change its shape |
| Cause some or ALL of these at once |
If something is sitting still, that doesn't mean NO forces act on it — it might mean the forces are balancing each other out perfectly.
🤝 A force needs TWO objects
When you push a table, your hand is one object applying a force on another object — the table. Your hand and the table are two objects interacting with each other. Look back at every action we've talked about — a bat hitting a ball, a hand lifting a bag, a magnet pulling a magnet — every single one of them involves TWO objects.
That's the big idea: at least two objects must interact for a force to come into play. A force is a push or pull on an object resulting from that object's interaction with another object.
When you push the table with your hand, do you feel a force on your hand too? Yes! Whenever two objects interact, each object feels a force from the other — the table pushes back on your hand just as you push on it. The moment you stop pushing (the interaction stops), that force on your hand disappears instantly. No contact, no force!
Contact forces
Contact forces only act when objects physically touch. Two big examples in this chapter:
The force from your muscles contracting and stretching — walking, running, lifting, pushing, jumping all use muscular force. Even your heart pumping blood and your stomach digesting food use muscular force internally!
🛑 Friction: the force that slows things down
Friction is the force that comes into play when an object moves (or tries to move) over another surface. It ALWAYS acts opposite to the direction of motion. Even "smooth" surfaces have tiny invisible bumps that lock together and resist sliding — that's what causes friction!
A ball rolls further on smooth tile than on rough sand. This is why ice is so slippery — it has very few surface bumps, so there's almost no friction to grip your shoes!
Air and water also create friction (drag) on things moving through them. That's exactly why aeroplanes, ships, and bullet trains are shaped sleek and pointed — to cut through the air/water with less resistance.
Non-contact forces
Non-contact forces act even without touching! There are three in this chapter:
| Force | What it does | Attract, repel, or both? |
|---|---|---|
| Magnetic force | Force between magnets/magnetic materials | Both (like poles repel, unlike attract) |
| Electrostatic force | Force between charged objects | Both (like charges repel, unlike attract) |
| Gravitational force | Earth pulling objects toward itself | ALWAYS attractive only! |
Unlike magnetic or electrostatic force, gravity never repels anything. It's always pulling objects together.
⚡ Static electricity in action
Rub a plastic scale (or a plastic straw) vigorously with a piece of polythene. Without touching the rubbed part, bring it close to some small pieces of paper lying on a table.
Rub two balloons with the same woollen cloth, then hang them close together.
Static charges come in two types: positive and negative. Same-type charges repel; opposite-type charges attract — the rubbing cloth and the rubbed object always end up with OPPOSITE charges, which is why they attract each other afterward.
🎮 Sort the forces: contact or non-contact?
Weight & floating
Mass = the amount of matter in an object (in kg) — stays the SAME everywhere. Weight = the force of gravity pulling on that object (in newtons, N) — CHANGES depending on where you are!
In daily life we often say things like "the weight of this wheat bag is 10 kg" — using a mass unit (kg) but calling it "weight"! That's how people talk casually, but it's scientifically incorrect. Kilogram is a unit of MASS, not weight. Weight should always be measured in newtons (N). In your science answers, always use the correct term with its correct unit!
📏 How to read a spring balance scale
A spring balance has a spring fixed at one end and a hook at the other. Hang an object on the hook and the spring stretches — the more it stretches, the greater the weight. A pointer moves along a marked scale (in newtons) to show the reading.
Suppose a spring balance's scale goes from 0 N up to a maximum of 10 N. The scale has big marks at every whole newton (0 N, 1 N, 2 N…), and between each pair of big marks there are 5 small divisions. What is the smallest weight this spring balance can read?
Not every spring balance is the same — one in your school lab might have a different maximum range or a different number of divisions between marks. Always count the divisions and work out the value of one small division first, before you take any reading, on a spring balance OR any other instrument.
🎮 Try the spring balance
| Planet/Moon | Weight of a 1 kg object |
|---|---|
| Earth | 10 N |
| Moon | 1.6 N |
| Mars | 3.8 N |
| Venus | 9 N |
| Jupiter | 25.4 N |
🛟 Floating and sinking
When you scoop water out of a bucket with a mug, the mug feels lighter while it's still under the water — and suddenly feels heavier the moment you lift it out! That's the water pushing UP on the mug while it's submerged. Let's find out what that upward push is.
When an object is placed in a liquid, the liquid pushes back up on it — this is called upthrust or buoyant force. If gravity pulling DOWN is stronger than the buoyant force pushing UP, the object sinks. If they're equal, it floats.
Pumice is a volcanic rock that can float on water! When lava full of trapped gas cools very quickly, it freezes with tiny gas bubbles inside — making it light and porous enough to float.
Every question from the book
(i) True — a change in speed requires force.
(ii) False — friction opposes motion and slows a ball down, never speeds it up.
(iii) False — charged objects exert electrostatic force on each other even without touching, since it's a non-contact force.
They move apart (repel). Rubbing both with the same cloth gives them the SAME kind of static charge, and like charges always repel each other.
For the coin, gravity (its weight) is greater than the buoyant force water can provide, so it sinks. For the wooden block, the buoyant force from displaced water balances its weight, so it floats. Wood is much less dense than metal — for its size, it weighs less, letting water support it.
Gravity acts downward the WHOLE time — it never switches off or changes direction!
(i) Going up: gravity pulls down, opposite to motion, so the ball slows down.
(ii) Coming down: gravity pulls down, same direction as motion, so it speeds up.
(iii) At the top: velocity is momentarily zero, but gravity is STILL pulling down — that's exactly why it starts falling again.
Smooth surfaces have very few bumps/irregularities, so there's very little friction between your shoes and the ground. Without enough friction to grip, there's nothing to stop your feet sliding — so you slip!
The Moon's gravity is much weaker than Earth's (about 1/6th) — since weight is the pull of gravity, weaker gravity means less weight.
No! Mass never changes. Mass is the amount of matter in an object, and that stays exactly the same wherever you go — on Earth, the Moon, or anywhere else. Only weight changes with gravity.
Yes. A force is exactly what changes an object's speed or direction — so if the motion is non-uniform (speed or direction is changing), a force MUST be acting on it.
(i) Increase friction on the flat surface — e.g. add sandpaper or cloth. More friction means more opposing force, so the ball stops sooner.
(ii) Decrease friction — e.g. make the surface smoother, or add oil. Less friction means the ball keeps rolling further before stopping.
(ii) w1 > w2 > w3
All three objects are floating — not sitting on the bottom of the container — so each one dips down until its weight is balanced by the buoyant force (the upward push of the water). Since all three are the SAME size and shape, the ONLY way an object can displace more water (and get more buoyant force) is by sinking deeper.
Object 1 dips the deepest of the three, so it needed the biggest buoyant force to hold it up — meaning it's the heaviest. Object 3 dips the least (floats highest), so it needed the smallest buoyant force — it's the lightest. That gives us w1 > w2 > w3.
Practice like the real exam
MCQ style (1 mark each)
(b) newton (N)
(c) Gravitational
Short answer (2 marks)
Longer answer (3 marks)
Mass is the amount of matter in an object, measured in kg, and never changes. Weight is the force of gravity pulling on that object, measured in newtons, and changes depending on where you are. For example, a 1 kg object weighs 10 N on Earth but only 1.6 N on the Moon — its mass (1 kg) is identical in both places, but its weight is very different because the Moon's gravity is much weaker.
Case study (4 marks)
(a) Ice is very smooth, with very few surface irregularities, so there's little friction and the skater glides easily. Sand is rough and uneven, creating much more friction that resists movement.
(b) Sand has more friction — its rough, uneven surface creates more resistance to sliding than ice's smooth surface.
(c) Low friction is useful for ice skates or ship hulls (less resistance = easier movement). High friction is useful for shoe soles or bicycle brakes (more grip = better control and stopping).
You did it! 🎉
These are fun bonus activities from the textbook (not exam questions, just for exploring!): rub different materials together and see which ones get charged · imagine a story where gravity disappears · debate "is friction a necessity or a problem?" · build your own spring balance · build a simple electroscope (jar + straw + copper wire + foil) to detect static charge.
🏁 Chapter 5 of 7 · Term 1 Science · Prishita, Class 8