Mission 1 of 4 — Newton’s First LawWhat keeps a moving
The rockets that launched Voyager 1 burned out in 1977. It is still crossing space at about 17 km/s.
What keeps a moving
thing moving?
The rockets that launched Voyager 1 burned out in 1977. It is still crossing space at about 17 km/s.One push, then let go. Predict first, then test it.
IGCSE 0625 · 1.5.1GCSE · ForcesAP Physics 1 · U2NGSS · HS-PS2-1Why seatbelts save lives
Predict first
You push a ball across perfectly smooth ice — no friction, no air resistance — then let go. What does it do?
Inertia lab One push, then let go
WaitingDrag the ball and let go — or press Push. Ice is selected.
What you just saw
Ice — nothing pushed back, so it kept the same speed. a flat line
Wood or carpet — friction pushed back, so it slowed down (and warmed both surfaces a little). slopes down to 0
A resultant force changes motion: the speed, the direction, or both. With none, nothing is needed to keep it moving — Voyager 1 has had no push since 1977 and still has its 17 km/s.
Newton’s First LawAn object stays at rest, or keeps moving at a steady speed in a straight line, unless a resultant force acts on it.
resultant force = 0 ⇒ velocity stays the same
resultantall forces added up (N)
velocityspeed + direction
The scenarioA small motorboat crosses a calm lake at a steady 8 m/s in a straight line. Its propeller gives a forward driving force of 1500 N. What is the total resistive force (water + air resistance)?
1
Spot the clue. “Steady speed” in a straight line across the lake → velocity isn’t changing → resultant force is 0 N (First Law).
2
Balance the line. With no resultant force, forward forces = backward forces.
3
Answer. Resistive forces add up to 1500 N, backward.
4
Check. 1500 N back against 1500 N forward leaves nothing over: no speeding up, no slowing down. That is “steady”. Units: N both sides.
Your turn — one step
A crate is pushed across a floor at a steady 1.5 m/s in a straight line, with a 90 N push.
1
Steady speed in a straight line, so the resultant force on the crate is N
2
So friction must balance the push: 90 N, backward.
Challenge
A cyclist and her bike have a total mass of 90 kg. She rides at a steady 8 m/s along a straight, flat road. Air resistance on her is 30 N, friction in the bike is 12 N, and a tailwind pushes her forward with 10 N. What forward force must she provide?
N
Steady speed in a straight line → resultant force is 0 N → everything forward = everything backward. Two forces push her backward; two push her forward, and the tailwind is one of them.
1Steady speed in a straight line → resultant force is 0 N (First Law). Her mass is not needed.
2Backward: 30 + 12 = 42 N. Forward: tailwind 10 N + her push.
3Her push + 10 = 42, so her push = 32 N.
4Check: with no wind she would need all 42 N; a tailwind should make it easier, and 32 < 42. Units: newtons throughout.
One more, on your own
A rowing boat moves at a steady 3 m/s in a straight line. Water resistance on it is 140 N and air resistance 25 N; the current pushes it forward with 40 N. What forward force do the oars provide?
N
Steady speed: forward forces = backward forces. Add the two backward forces; the current already supplies part of the forward side.
1Backward: 140 + 25 = 165 N.
2Forward: oars + 40 N = 165 N.
3Oars = 125 N.
Mission 2 of 4 — Newton’s Second LawSame push.
An empty trolley rolls off with a nudge. A full one barely budges.
Same push.
Different mass.
An empty trolley rolls off with a nudge. A full one barely budges.Push a box for one second, then again with twice the mass.
IGCSE 0625 · 1.5.1GCSE · ForcesAP Physics 1 · U2NGSS · HS-PS2-1Why race cars are built light
Predict first
You’ll push the box with a set force, then double its mass and push again with the same force. What happens to how quickly it speeds up?
Force lab A 1-second push on a frictionless track
WaitingPick a force and a mass, then click the box — or press Push.
Read your graph
Your last run is the bright line. On a speed–time graph the slope is the acceleration, and a flat line means a steady speed. Drag across the graph if you want to check a point.
1
How fast was the box moving at t = 0.5 s, half-way through the push? m/s
2
And at t = 1.5 s, half a second after the push ended? m/s
What the box showed you
Double the mass → half the acceleration. The full trolley gets the same nudge as the empty one, with more mass, so it barely moves. half as steep
More force → more acceleration. steeper
Push over → steady speed. The First Law again. flat after 1 s
Newton’s Second LawA resultant force makes an object accelerate in the direction of the force. More force → more acceleration; more mass → less acceleration.
F=m×a
Fresultant force (N)
mmass (kg)
aacceleration (m/s²)
The scenarioA 1200 kg car accelerates at 3 m/s². What resultant force does that take?
1
Pick the law. A mass and an acceleration, and we want a force: F = m × a.
2
Substitute. F = 1200 × 3.
3
Answer. F = 3600 N, in the direction of the acceleration. Add passengers and the same force gives less acceleration.
4
Check. 3600 N is about a third of the car’s weight (≈ 11 800 N), and 3 m/s² is about a third of g. They agree. Units: kg × m/s² = N.
Now you try — fill the gap
A resultant force of 600 N acts on a 150 kg motorbike and rider. Find the acceleration.
1
Rearrange F = m × a for acceleration: a = F ÷ m.
2
a = 600 ÷ 150 = m/s²
3
Every second the bike gains 4 m/s of speed while that force acts.
Challenge
A car’s engine gives a driving force of 2400 N. Friction and air resistance on the car total 900 N. The car accelerates at 1.0 m/s². What is its mass?
kg
Two forces act along the road, so find the resultant first. Then rearrange F = m × a so that m is on its own.
1Resultant force: 2400 − 900 = 1500 N, forward.
2Rearrange: m = F ÷ a = 1500 ÷ 1.0.
3m = 1500 kg.
4Check: 1500 kg is a family car. Without resistance the same engine would give 1.6 m/s², so 1.0 with resistance is right. Units: N ÷ (m/s²) = kg.
One more, on your own
A van has a driving force of 3000 N and 1200 N of resistance. It accelerates at 1.5 m/s². What is its mass?
kg
Resultant first: driving force minus resistance. Then m = F ÷ a.
1Resultant: 3000 − 1200 = 1800 N.
2m = 1800 ÷ 1.5.
3m = 1200 kg.
Mission 3 of 4 — Resultant ForceWhat happens to a
Skydivers stop speeding up at about 200 km/h — even though gravity never switches off.
What happens to a
falling skydiver’s speed?
Skydivers stop speeding up at about 200 km/h — even though gravity never switches off.Gravity pulls down, the air pushes up. Pick an answer, then jump.
IGCSE 0625 · 1.2 & 1.3GCSE · ForcesAP Physics 1 · U2NGSS · HS-PS2-1Why raindrops don’t hurt
Predict first
A skydiver jumps from a plane with her parachute closed. Air resistance on her grows as she speeds up. What happens to her speed?
Freefall lab Weight vs air resistance
WaitingSet her mass, then click her — or press Jump. Watch the two arrows.
Three discoveries 0 / 3
Top speedFall until her speed stops changing
MassChange her mass by at least 15 kg and reach top speed again
ParachuteOpen the parachute and wait until her speed is steady again
Read your graph
Your last jump is the bright line. Where it climbs steeply she is gaining speed fast; where it is flat the forces balance.
1
In the first 2 s, by about how much did her speed grow each second? (Read the speed at 2 s and divide by 2.) m/s²
2
By how much did the parachute cut her steady speed? Read both flat parts of the line and subtract. m/s
What the jump showed you
At first — weight wins, so she speeds up. climbs
Forces balance → steady speed: terminal velocity, about 200 km/h for a skydiver. levels off
Heavier, same size and shape → a higher terminal velocity. Every jump starts with the same acceleration, g. same start, levels off higher
Parachute — air resistance wins, so she slows down (still falling). drops to a new level
Terminal velocityWhen the forces on a falling object balance, the resultant force is zero and its speed stops changing.
resultant force = weight − air resistance
W=m×g
Wweight (N)
mmass (kg)
g9.8 N/kg on Earth
down +take down as positive: a negative resultant points up
The scenarioA 70 kg skydiver is falling. At one moment the air resistance on her is 400 N. Find the resultant force and her acceleration.
1
Weight first. Take down as positive. W = m × g = 70 × 9.8 = +686 N.
2
Resultant along the line. 686 − 400 = +286 N. Positive, so down — weight wins, and she is still speeding up.
3
Second Law. a = F ÷ m = 286 ÷ 70 = 4.1 m/s², down.
4
Check. 4.1 m/s² is less than g = 9.8 m/s², as it must be: the air is already taking some of the pull away. Units: N ÷ kg = m/s².
Over to you — one blank left
A 60 kg skydiver has reached terminal velocity.
1
Her weight: W = m × g = 60 kg × 9.8 N/kg.
2
At terminal velocity, the air resistance on her is N
3
Air resistance = weight = 60 × 9.8 = 588 N, so the resultant force is 0 and her speed is steady.
+15 XP bonus
Rare challengeCombine it all
An 80 kg skydiver opens her parachute. A moment later the air resistance on her is 1200 N. What is her acceleration — and which way does it point?
m/s²
Start with her weight: W = m × g. Then compare it with the 1200 N of air resistance — which force is bigger, and by how much?
1Take down as positive. Weight: W = 80 × 9.8 = +784 N.
2Resultant: 784 − 1200 = −416 N. Negative, so up: air resistance wins.
3a = −416 ÷ 80 = −5.2 m/s², that is 5.2 m/s², up. Still falling — but slowing down.
4Check: about half of g. A parachute that slows you at half a g the moment it opens is believable; 50 m/s² would not be. Units: N ÷ kg = m/s².
One more, on your own
A 90 kg skydiver has just opened her parachute. The air resistance on her is 1242 N. What is her acceleration, and which way does it point?
m/s²
Weight first (W = m × g), then the resultant (which force is bigger, and by how much?), then a = F ÷ m.
1Weight: 90 × 9.8 = 882 N, down.
2Resultant: 1242 − 882 = 360 N, up.
3a = 360 ÷ 90 = 4.0 m/s², up.
Mission 4 of 4 — Newton’s Third LawYou push a wall.
In empty space a rocket has nothing to push against — yet it speeds up.
You push a wall.
What does the wall do?
In empty space a rocket has nothing to push against — yet it speeds up.Start simpler: lean on a wall and push hard.
Beyond IGCSE 0625GCSE · ForcesAP Physics 1 · U2NGSS · MS-PS2-1How every step pushes the Earth
Predict first
You push on a wall with a force of 200 N. What does the wall do to you?
Pair finder Find each force’s partner
WaitingFind the partner of the coloured force: tap a white arrow, or an answer.
What every pair had in common
Every force has a partner: A on B ↔ B on A.
Same size, opposite direction, same type of force.
They act on two different objects, so they never cancel each other out.
Newton’s Third LawWhen object A exerts a force on object B, B exerts a force on A that is equal in size, opposite in direction and of the same type.
FA on B=−FB on A
=same size
−opposite way
A, Btwo different objects
The scenarioA rocket engine pushes exhaust gas backward with 30 000 N. What force does the gas exert on the rocket — and in which direction?
1
Name the pair. Rocket pushes gas ↔ gas pushes rocket.
2
Same size, opposite direction. The gas pushes the rocket forward with 30 000 N.
3
Why it doesn’t cancel. One force acts on the gas, the other on the rocket. Only the force on the rocket goes into its resultant — that’s what launches it, even in empty space.
4
Check. Same force, very different masses: the rocket (tonnes) gains speed slowly, the gas (kilograms) is thrown back at kilometres per second. a = F ÷ m, both ways.
Your move — pick a direction
When you walk, your foot pushes the ground backward.
1
So the ground pushes your foot ? tap one
2
That forward push from the ground is what moves you. It comes from friction — and ice has very little, so your foot slides back instead of pushing. That’s why walking on ice is so hard.
Final challenge
A 60 kg swimmer pushes the water backward with 150 N. Drag from the water on her is 110 N. What is her acceleration?
m/s²
Start with the third law: she pushes the water backward with 150 N — what does the water do to her? Then find the resultant force on her.
1Third law: the water pushes her forward with 150 N.
2Resultant on her: 150 − 110 = 40 N, forward.
3Second law: a = 40 ÷ 60 = 0.67 m/s², forward. (Her push on the water acts on the water, so it never enters her resultant.)
4Check: 0.67 m/s² is small, as swimming is — a couple of seconds to reach walking pace. Units: N ÷ kg = m/s².
One more, on your own
A 75 kg swimmer pushes the water backward with 180 N. Drag from the water on her is 120 N. What is her acceleration?
m/s²
Third law for the force the water puts on her, then subtract the drag, then a = F ÷ m.
1The water pushes her forward with 180 N.
2Resultant: 180 − 120 = 60 N, forward.
3a = 60 ÷ 75 = 0.8 m/s², forward.