Newton's three laws of motion · free-body diagrams · F = ma · friction · inclined planes
A force is only a push or a pull — and motion does not need one. What needs a force is a change in motion. Add up every force on an object and you get one leftover arrow; that arrow, divided by the mass, is the whole story of how things speed up, slow down and turn.
This page covers Newton's three laws of motion — inertia, F = ma, and action–reaction — together with free-body diagrams, friction (static vs kinetic) and inclined planes. By the end you'll be able to:
Every motion you meet — a braking bus, a launching rocket, a single walking step — obeys these three rules. They are the first thing every physics course builds on. We go beyond the syllabus, but we never skip it:
Searched as: newton's laws of motion, free body diagram, F = ma problems, laws of motion formulas, static vs kinetic friction.
Pick a scenario, set the masses and forces, and the machine draws every force as an arrow — then solves it the way a teacher would on a board. 🟢 real free-body engine
Pick a scenario and press Calculate forces to see the worked solution.
In plain terms: the first law (inertia), the second law (F = ma), and the third law (action–reaction) — then how the calculator puts all three to work.
A force is simply a push or a pull. Newton's real insight was that an object only changes how it moves — speeds up, slows down, or turns — when the forces on it do not cancel. Draw every force acting on a body as an arrow and add them, and you are left with one arrow: the net force. If that leftover is zero, the body keeps doing exactly what it was already doing. If it is not zero, the body accelerates in the direction the leftover points. Motion itself needs no cause; only a change in motion does.
Three laws carry the whole story. The first says a body left alone keeps its velocity — this stubbornness is inertia, and heavier bodies have more of it. The second puts a number on it: net force equals mass times acceleration, F = ma, so the same push gives a light cart a big acceleration and a loaded truck a small one. The third says forces come in pairs — push on a wall and the wall pushes back equally, on you. To use them you isolate one object, draw every force touching it, split those forces along sensible axes, and apply F = ma on each axis. Solve, and the motion falls out.
The machine above is a free-body engine. Pick a scenario — a block pulled on a flat surface, a block on an incline, or two masses over a pulley — and it draws every force as a coloured arrow: weight pulling down, the normal force pushing up off the surface, your applied force, and friction resisting the slide. It checks whether your push beats the maximum static friction; if it does, the block breaks free and kinetic friction takes over. Then it resolves weight into components along and across the incline, balances the perpendicular direction to find the normal force, and runs F = ma along the slope to report the acceleration. 🟡 maths of the picture
The physics is visible in the diagram; the maths usually hides. These little labs make it visible too — drag a slider and watch the numbers and arrows answer.
In a sudden stop the car slows fast, but your body — obeying the first law — keeps moving forward at the old speed. The belt is the outside force that finally slows you with the car instead of into the dashboard.
A rocket hurls hot gas downward; by the third law the gas pushes the rocket up with equal force. It needs nothing to push against — only its own exhaust — which is why it works in the vacuum of space.
You walk by pushing backward on the ground; friction pushes you forward. On smooth ice that grip vanishes, your foot slips, and the same step that carried you forward now just slides in place.
Physics you can hold. Each project below demonstrates a law you just met — and the measuring is what turns a demo into a science-fair winner. Pick one, build it from things at home, and graph something.
Build: thread a straw onto a long fishing line across the room; tape an inflated balloon to the straw and let go — air rushes back, the car shoots forward.
Measure: distance travelled for each balloon size, then graph distance against the air you let out.
Build: roll a toy cart with an egg "passenger" into a wall — once strapped down, once loose.
Measure: the speed at which the belt stops saving the egg. Inertia made visible (and messy).
Build: pump air into a part-filled soda bottle on a launch stand; water blasts down, the bottle climbs.
Measure: peak height for each water level — find the fill that flies highest, and explain why.
Build: hang two unequal masses over a low-friction pulley with string.
Measure: time the fall for different mass pairs and test a = (m₁−m₂)g ÷ (m₁+m₂) against your stopwatch.
Build: slowly tilt a board with an object on it until it just begins to slide.
Measure: the tilt angle gives μₛ = tan θ — compare wood, rubber, paper and rank the surfaces.
Build: glue a pop-top cap over a CD's hole and attach a balloon; escaping air floats the disc almost frictionlessly.
Measure: how far one gentle push sends it gliding — the first law with friction nearly switched off.
Newton's laws sit under almost every motion you see. Here are the questions that come up most — each answer reads on its own, lifted clean off the page.
Newton's second law states that the acceleration of an object equals the net force on it divided by its mass, written F = ma. To use it, add every force on the object as a vector to find the net force, then divide by the mass to get the acceleration. The same push gives a light cart a large acceleration and a heavy truck a small one, because acceleration is inversely proportional to mass.
Mass is the amount of matter in an object and stays the same everywhere, measured in kilograms. Weight is the gravitational force pulling that mass down, measured in newtons, and equals mass times g (about 9.81 m/s² on Earth). A 5 kg bag always has a mass of 5 kg, but it weighs about 49 N on Earth and far less on the Moon, where g is smaller.
Passengers lurch forward when a car stops suddenly because of inertia, described by Newton's first law. The car decelerates, but the passengers' bodies keep moving forward at the original speed until an outside force slows them. That force is the seatbelt; without it, the body keeps going until it hits the dashboard or windscreen.
A free-body diagram is a sketch of one chosen object showing only the external forces acting on it, each drawn as an arrow. Typical forces are weight (down), the normal force (perpendicular to the surface), applied pushes or pulls, friction (opposing sliding) and tension (along strings). Internal forces and forces the object exerts on other things are never drawn, because the diagram analyses just that one body.
Friction decides whether an object moves by comparing your applied force with the maximum static friction, which equals the static coefficient times the normal force. While your push is smaller than this maximum, static friction grows to exactly cancel it and the object stays still. Once your push exceeds the maximum, the object breaks free and slides, and the usually smaller kinetic friction takes over.
An object on an inclined plane feels three main forces: its weight pulling straight down, the normal force pushing perpendicular to the slope, and friction acting along the slope. The weight is split into a component down the slope, mg·sin(θ), and a component into the slope, mg·cos(θ). The normal force balances mg·cos(θ), while mg·sin(θ) is what tries to slide the object down.
Newton's third law states that for every action force there is an equal and opposite reaction force. Action-reaction pairs always act on two different objects: when you push on a wall, the wall pushes back on you with the same size of force in the opposite direction. Because the two forces act on different bodies, they never cancel each other out.
| Surface | Normal force | Why |
|---|---|---|
| Flat, no vertical push | N = mg | It balances the full weight |
| Incline at angle θ | N = mg·cos(θ) | It balances only the perpendicular part of weight |
| Vertical wall (90°) | N = 0 | cos(90°) = 0 — nothing to lean on |
A rocket accelerates in empty space because of Newton's third law. It throws hot exhaust gas backward at high speed, and the gas pushes the rocket forward with an equal and opposite force called thrust. The rocket does not need air or ground to push against — the reaction to expelling its own fuel is enough — which is exactly why rockets work in the vacuum of space.
Seven question formats, the way Beyond Dictionary serves them — multiple choice, multiple-correct, fill-in-the-blank, match, sequence, read-think-connect, and write-your-own. Every question has layered hints: a quick nudge, the reasoning, then a deeper connection — so a wrong answer opens a door, never a dead end. 🟢 received from a board-tagged question bank · seed toward 2,000
Pick your board — the set re-tunes to its wording and emphasis. Competitive draws the JEE / NEET / Olympiad lane.
Force is the quiet hand behind every change of motion — push the world, and it pushes back.