Force and Laws of Motion
Every push and pull is a force — and this lab is all about how forces affect motion. You'll balance a tug of war, feel inertia jerk a bus passenger, launch a balloon rocket, and prove momentum is conserved, before testing yourself with flashcards, quizzes and a 2-minute Bubble Burst round.
Newton's 3 Laws
Explained through 9 hands-on simulations you can drive with sliders and buttons.
Real formulas
Momentum, force, acceleration and recoil velocity — with worked numericals.
Quiz & recall
Flashcards, MCQs, fill-in-the-blanks and a timed Bubble Burst game.
What is a force?
A force is simply a push or a pull that one object exerts on another. You can't see a force directly — only its effects.
Starts motion
A force can move an object that was at rest — like kicking a football.
Stops motion
A force can stop a moving object — like catching that same ball.
Changes speed
A force can speed up or slow down a moving object.
Changes direction
A force can change the direction an object is moving in — a batsman deflecting a ball.
Changes shape
A force can change an object's shape or size — squeezing a sponge.
Force ⇄ Motion
We now study exactly how — with three precise laws given by Sir Isaac Newton.
Balanced forces
When the forces acting on an object combine to give a net (resultant) force of zero, they are called balanced forces.
No change in motion
- A body at rest stays at rest.
- A moving body keeps moving the same way — same speed, same direction.
- Balanced forces can still change an object's shape.
Evenly matched tug-of-war
Two teams pull a rope with exactly equal force in opposite directions. The rope doesn't move either way — the pulls are balanced, even though both teams are clearly applying force.
Unbalanced forces
When forces on an object do not cancel out, a net force remains — and that net force changes the object's motion.
Motion changes
- An object at rest can start moving.
- A moving object can speed up, slow down, or change direction.
- In short: an unbalanced force produces acceleration.
An unequal tug-of-war
If one team pulls harder than the other, the net force is no longer zero — the rope (and both teams) move toward the stronger side.
Push battle — balanced vs unbalanced
Two people push a crate from opposite sides. Change each push and watch the crate respond to the net force.
Meet Galileo & Newton
Before this chapter existed, two scientists changed how we think about motion forever.
Galileo Galilei
By rolling balls down inclined planes, Galileo argued that a moving object would keep moving forever if nothing (like friction) acted to slow it down. This idea — motion needs no force to continue — was revolutionary.
Sir Isaac Newton
Newton built on Galileo's idea and published three precise laws connecting force and motion in his book Principia Mathematica. They still describe everyday motion perfectly.
Newton's First Law of Motion
Nothing changes on its own
A book on a table stays put forever unless something pushes it. A hockey puck on frictionless ice would slide in a straight line forever unless something acted on it.
The Law of Inertia
This law is so linked to the idea of "resisting change" that it's commonly called the Law of Inertia — our next big idea.
What is inertia?
Inertia is the natural tendency of an object to resist any change in its state of rest or of uniform motion in a straight line.
Objects "want" to keep doing what they're doing
- A resting object "wants" to stay at rest.
- A moving object "wants" to keep moving the same way.
- It takes an unbalanced force to change either.
Inertia is NOT a force
Inertia is a property of matter (related to mass) — not something that pushes or pulls. It's the reason a force is needed to change motion, not a force itself.
Three types of inertia
| Type | What it means | Example |
|---|---|---|
| Inertia of Rest | A body at rest tends to stay at rest | Passengers jerk backward when a bus starts suddenly |
| Inertia of Motion | A moving body tends to keep moving | Passengers jerk forward when a bus stops suddenly |
| Inertia of Direction | A body moving one way tends to keep that direction | Mud flies off tangentially from a spinning cycle wheel |
Inertia bus lab 🚌
Watch the standing passenger react as the bus changes speed. No hands-on-rail here — just inertia at work!
Mass is the measure of inertia
Not all objects resist change equally — the more mass an object has, the more inertia it has, and the harder it is to start, stop, or turn it.
Empty vs loaded cart
It's far easier to push an empty shopping cart than one piled high with groceries — the loaded cart has more mass, so more inertia, so it resists your push more.
Truck vs bicycle
A truck moving at 20 km/h is much harder to stop than a bicycle at the same speed — the truck's huge mass gives it huge inertia of motion.
Heavy vs light — same push
Apply the exact same 20 N force to a light box and a heavy box. Which one budges more?
Inertia in everyday life
🧹 Dusting a carpet
When you beat a hung carpet, the carpet moves suddenly but the dust particles' inertia of rest keeps them behind — so they fall out.
🌳 Shaking a mango tree
Shaking the trunk moves the branches, but the mangoes' inertia makes them lag, snapping their stalks and falling off.
🔨 Tightening a hammer head
Striking the handle's bottom on the ground stops the handle suddenly; the heavier head keeps moving down due to inertia, wedging on tighter.
🚗 Seatbelts
In a sudden stop, your body's inertia of motion carries you forward — a seatbelt supplies the unbalanced force to stop you safely.
What is momentum?
Momentum measures the "quantity of motion" in a moving body — how hard it would be to stop it. It depends on both how heavy something is and how fast it's going.
kg·m/s
Sometimes written as newton-second (N·s) — both mean the same thing.
Direction matters
Momentum points in the same direction as velocity. A truck moving north and one moving south don't just add their momenta — direction counts.
Momentum comparator
Slide the mass and velocity of a truck and a bicycle, and watch which one is harder to stop.
Newton's Second Law of Motion
Bigger force, bigger change
Push harder on something and its momentum (and usually its speed) changes faster. This law connects force directly to how motion changes.
Toward a formula
This statement can be turned into precise maths — the most useful equation in the whole chapter. Let's derive it.
Deriving F = ma
Let's turn the second law's words into the formula you'll use constantly.
From momentum to force
- An object of mass m moves with initial velocity u. Initial momentum = mu.
- A force acts for time t, changing velocity to v. Final momentum = mv.
- Change in momentum = mv − mu = m(v − u).
- Rate of change of momentum = m(v − u) / t.
- Since acceleration a = (v − u) / t, this becomes m × a.
By Newton's Second Law
Force ∝ rate of change of momentum, so F ∝ ma. In SI units, the constant of proportionality is exactly 1 — giving us the clean formula:
The newton — unit of force
Force is measured in an SI unit named after the man himself.
1 N = 1 kg·m/s²
Any time you compute F = ma with kilograms and metres per second squared, your answer comes out in newtons automatically.
Bigger than you'd think
Lifting an average apple against gravity takes about 1 N of force — a handy way to picture the size of a newton.
F = ma playground
Set the mass and the applied force — watch the resulting acceleration play out on the track.
Solved numerical — Second Law
Find the force needed to accelerate a 1500 kg car at 2 m/s².
- Given: mass m = 1500 kg, acceleration a = 2 m/s²
- Formula: F = m × a
- Substitute: F = 1500 × 2
- Answer: F = 3000 N
Law 1 is a special case of Law 2
The two laws you've just learned aren't separate facts — they fit together perfectly.
If F = 0…
From F = ma, if F = 0 then a = 0 too (since mass is never zero). Zero acceleration means velocity never changes.
Newton's First Law!
"Velocity never changes unless a force acts" is precisely the Law of Inertia. The first law is simply the second law in the special case where the net force is zero.
Why we pull hands back catching a ball
A fast cricket ball carries a lot of momentum. Catching it changes that momentum to zero — but how quickly you do that changes the force you feel.
Stop fast → huge force
If you hold your hands still, the ball stops in a very short time. Since F = Δp/Δt, a small Δt means a large force — it stings!
Stop slow → gentle force
Pulling your hands back as you catch increases the stopping time. The same change in momentum now spreads over a longer Δt, so the force is much smaller.
Catch the ball — same Δp, different force
Try both catching styles for the same incoming ball and compare the force each one produces.
More Second Law in action
🥋 Karate chop
A karate expert strikes a slab with maximum speed, stopping their hand in a very short time — producing a huge force that shatters the slab.
🚗 Crumple zones & airbags
Cars are built to crumple and airbags inflate to increase the collision time — reducing the force on passengers for the same speed change.
🤸 Bending knees on landing
Athletes bend their knees when landing a jump, extending the stopping time and easing the force on their joints.
📦 Padding & straw
Fragile items are packed with straw or bubble wrap so a fall's momentum change happens over a longer time, lowering the impact force.
Newton's Third Law of Motion
Forces always come in pairs
Whenever object A pushes on object B (the action), object B pushes back on A with equal strength in the opposite direction (the reaction) — at exactly the same instant.
Different bodies!
The action acts on one object; the reaction acts on the other. They never act on the same object, which is why they don't simply cancel out.
Balloon rocket lab 🎈
Let air rush out of a balloon and watch the balloon shoot the opposite way — action and reaction, live.
Everyday action–reaction
| Situation | Action | Reaction |
|---|---|---|
| Walking | Foot pushes ground backward | Ground pushes foot forward — you move! |
| Swimming | Hands push water backward | Water pushes swimmer forward |
| Firing a gun | Gun pushes bullet forward | Bullet pushes gun backward (recoil) |
| Rowing a boat | Oar pushes water backward | Water pushes boat forward |
| Rocket launch | Rocket pushes hot gas downward | Gas pushes rocket upward |
Why action & reaction don't cancel
If action and reaction are equal and opposite, why does anything ever move? The answer is where each force acts.
Two different bodies
The action acts on Body B; the reaction acts on Body A. Since they act on different objects, each object still feels only one of the two forces — so each can still accelerate.
Same body
Balanced forces act on the same object, which is why those genuinely cancel out for that object. Don't mix the two ideas up!
Conservation of momentum
Newton's third law leads to one of the most powerful ideas in physics.
An isolated system
Take two objects that interact only with each other (a collision, an explosion, a gun firing) with no outside force involved.
Total momentum never changes
The total momentum of the system before the interaction is exactly equal to the total momentum after it.
The formula, explained
m₁, m₂, u₁, u₂
Masses of the two objects, and their velocities before the collision or explosion.
v₁, v₂
The same two objects' velocities after the event. The total momentum on both sides always balances.
Gun & bullet recoil lab
Both start at rest, so their total momentum starts at zero — and must stay zero.
Two-cart collision lab
Cart A rolls into stationary Cart B and they stick together. Set the values and check: momentum before really does equal momentum after.
Solved numerical — conservation of momentum
A bullet of mass 0.02 kg is fired from a 4 kg gun with velocity 200 m/s. Find the gun's recoil velocity.
- Given: m₁ (bullet) = 0.02 kg, m₂ (gun) = 4 kg, v₁ (bullet) = 200 m/s. Both start at rest, so total initial momentum = 0.
- Formula: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂, with u₁ = u₂ = 0
- So: 0 = (0.02 × 200) + (4 × v₂)
- Solve: 4 × v₂ = −4 → v₂ = −1 m/s
- Answer: The gun recoils at 1 m/s, in the direction opposite the bullet.
All three laws, side by side
| Law | Short statement | Key idea | Example |
|---|---|---|---|
| First | Object keeps its state unless an unbalanced force acts | Law of Inertia | Bus jerk |
| Second | Rate of change of momentum ∝ unbalanced force | F = ma | Pushing a cart |
| Third | Action = − Reaction, on different bodies | Forces occur in pairs | Walking, rockets |
Formula cheat-sheet
p = mv
SI unit: kg·m/s. A vector — direction matters.
F = ma
SI unit of F: newton (N) = kg·m/s².
1 N = 1 kg·m/s²
Force that gives 1 kg an acceleration of 1 m/s².
m₁u₁+m₂u₂ = m₁v₁+m₂v₂
Total momentum before = total momentum after (isolated system).
Common mistakes & must-remembers
- Balanced ≠ no forces — they just add up to zero net force.
- Inertia is not a force — it's a property related to mass, not a push or pull.
- Action–reaction pairs act on different objects — they never cancel for a single body.
- Momentum is a vector — always assign a direction (sign) before adding or subtracting.
- A body can have zero net force and still be moving — it just won't accelerate (Law 1).
- Don't confuse mass (amount of matter, constant) with weight (force of gravity, can change).
Key terms to remember
| Term | Meaning |
|---|---|
| Force | A push or a pull exerted by one object on another. |
| Balanced forces | Forces whose resultant is zero; no change in motion. |
| Unbalanced forces | Forces whose resultant is not zero; produce acceleration. |
| Inertia | An object's natural resistance to a change in its state of rest or motion. |
| Momentum | The quantity of motion in a body; p = mv. |
| Newton (N) | SI unit of force; 1 N = 1 kg·m/s². |
| Recoil | The backward motion of a gun (or similar) after firing, from Law 3. |
| Conservation of momentum | Total momentum of an isolated system stays constant. |
Flashcards 🃏
Test yourself before you read the answer — then tap the card to check.
Multiple choice quiz ✍️
Pick one answer per question. You'll get instant feedback and a running score.
Fill in the blanks 🖊️
Type your answer, then press Check. Spelling is forgiving of capitals and spaces.
True or False? ⚡
Tap True or False for each statement.
Spot the law 🔎
For each everyday scene, choose which of Newton's laws explains it.
Bubble Burst 🫧⚡
A question appears above the tank. Pop the one bubble with the correct answer before it floats away — quick, right ones score more; wrong pops cost you.
Race the clock for 2 minutes. Tap the bubble that answers the prompt correctly — wrong pops cost 3 points, correct pops earn 10.
The whole chapter in bubbles 🫧
Everything connects back to Newton's three laws. Let these floating bubbles jog your memory.
Chapter complete!
Here's your one-screen takeaway. If you can explain each line to a friend, you know this chapter cold.
Remember these
- Force is a push or pull; balanced forces cancel, unbalanced forces don't.
- Inertia resists change in motion; mass measures it.
- Momentum p = mv is the quantity of motion.
How force shapes motion
- 1st: no change without an unbalanced force.
- 2nd: F = ma — force sets the rate of change.
- 3rd: every action has an equal, opposite reaction — and momentum is conserved.