Free Interactive Physics Demo — Newton's Laws (Forces & Motion)

Mission 1 of 3 — Newton’s Third Law

Every push
pushes back.

Drop the ball and watch the instant it lands. The ball pushes on the ground — and the ground pushes straight back. That second arrow is the reaction force.

IGCSE 0625 · 1.5GCSE · ForcesAP Physics 1 · U2NGSS · HS-PS2

COLLISION LAB Action & Reaction

Waiting

Gravity — 1

Bounciness — 0.8

What you just saw

The ball pushed down on the ground; the ground pushed back up just as hard. Forces always come in pairs — equal in size, opposite in direction, acting on different objects.

F_{A on B} = −F_{B on A}

Fforce between two objects (N)

−equal magnitude, opposite direction — always

The scenario A rocket engine pushes exhaust gas backward with a force of 30 000 N. What force does the gas exert on the rocket — and in which direction?

Solution — Newton’s Third Law We’re asked for a reaction force to a known push, so this is Newton’s Third Law.
Step 1 — the action: rocket pushes gas backward, 30 000 N.
Step 2 — the reaction is equal and opposite: gas pushes the rocket forward with 30 000 N.
Step 3 — the pair acts on different objects, so the forces don’t cancel. That’s what launches the rocket. Your turn below.

CHALLENGE Action–Reaction

A swimmer pushes back on the water with 150 N. How hard does the water push the swimmer forward? (Newton’s Third Law)

The swimmer pushes the water backward with 150 N, so the water pushes the swimmer forward with 150 N — equal size, opposite direction. That reaction force is what moves the swimmer through the pool.

Drop the ball to see the reaction

Why anything speeds up at all

+20 XP

Here’s the puzzle: if every force has an equal partner, how does anything ever speed up? The answer is a resultant force — a push that isn’t cancelled out. Let’s see it.

Module progress: 6% → 12%

Mission 2 of 3 — Newton’s Second Law

Same push.
Different mass.

Fire the box, then increase the mass and fire again with the same force. Watch the thrust trail stretch or shrink — that’s the acceleration changing.

IGCSE 0625 · 1.5GCSE · ForcesAP Physics 1 · U2NGSS · HS-PS2-1

Predict first

You’ll fire the box, then double the mass and fire again with the same force. Before you do — what happens to how fast it speeds up?

FORCE LAB F = m × a

Waiting

Force — 50 N

Mass — 10 kg

What you just saw

It’s tempting to think a moving object needs a constant force to keep going. It doesn’t. With no resultant force it just keeps its speed — that’s Newton’s First Law. A resultant force doesn’t maintain motion; it changes it, causing acceleration in the direction of the force. More force, more acceleration; more mass, less. That’s Newton’s Second Law: F = m × a.

F = m × a

Fnet force in newtons (N)

mmass in kilograms (kg)

aacceleration in metres per second squared (m/s²)

The scenario A 1200 kg car accelerates at 3 m/s². What resultant force is needed to do this?

Solution — F = ma We know a mass and an acceleration and want the force, so we use F = ma.
Step 1 — write it: F = m × a.
Step 2 — substitute: F = 1200 × 3.
Step 3 — calculate: F = 3600 N, in the direction of the acceleration. Add passengers and the same force gives less acceleration — mass resists the change. Your turn below.

CHALLENGE F = m × a

A 200 kg box speeds up at 4 m/s². What net force is pushing it?

Use F = m × a. Substitute: F = 200 × 4 = 800 N, acting in the same direction as the acceleration.

Fire the box to see acceleration

So far, one clean push. In the real world, forces act together — engines, gravity, friction and drag, all at once. What decides the motion then? The resultant of them all.

Module: 12% → 18%

Mission 3 of 3 — Net Force

Gravity never
lets go.

Set the angle and speed, then launch. The trail shows the path — and gravity keeps pulling down the whole way, bending every shot back to Earth.

IGCSE 0625 · 1.5GCSE · ForcesAP Physics 1 · U1–U2NGSS · HS-PS2-1

LAUNCH LAB Projectile & Net Force

Waiting

Angle — 45°

Speed — 50 m/s

What you just saw

Sideways speed stayed steady — but gravity (the purple arrow) kept pulling down, bending the path. A resultant force can change speed, direction, or both. Add the forces along a line and what’s left is the resultant force that decides the motion.

F_net = F_applied − F_friction

F_netthe force that actually accelerates the object

F_appdriving force (engine, push, thrust)

F_fresistance force (friction, drag)

The scenario A car engine drives it forward with 1200 N. Air resistance — the drag a gas exerts on anything moving through it — pushes back with 300 N. What’s the resultant force, and what does it do?

Solution — F_net = F_app − F_friction Two forces act along one line in opposite directions, so we find the resultant by subtracting.
Step 1 — subtract backward from forward.
Step 2 — F_net = 1200 − 300 = 900 N forward. That accelerates the car.
Step 3 — as speed rises, drag grows until F_net = 0 — then the car holds a steady speed. Friction works the same way, and heats the surfaces too. Your turn below.

✦ RARE

Combine it all

Bring it all together: the 1200 kg car’s engine drives it forward with 3000 N. Air resistance pushes back 400 N and friction another 200 N. Using F = m × a, what acceleration does the net force give the car?

m/s²

Step 1 — resultant force: 3000 − 400 − 200 = 2400 N forward.
Step 2 — rearrange: a = F ÷ m.
Step 3 — substitute: a = 2400 ÷ 1200 = 2 m/s².

Launch to trace the path

What you should remember

Five ideas that govern every motion.

✓Forces come in pairs — equal, opposite, and acting on different objects.

✓No resultant force — an object stays at rest or keeps a steady speed in a straight line.

✓A resultant force accelerates an object in the direction of the force: F = m × a.

✓Along one line: add forward forces, subtract backward ones — the resultant decides the motion.

✓Friction and drag oppose motion; friction between surfaces also produces heating.

Dynamics

Module: 18% Quiz → 24%

LESSON COMPLETE

Opening summary in 10s…

Every pushpushes back.

Same push.Different mass.

Gravity neverlets go.

Dynamics — Lesson 1: Forces & Motion

Every push
pushes back.

Same push.
Different mass.

Gravity never
lets go.