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Edexcel International GCSE Physics · Section 1 Forces and motion
Balanced and Unbalanced Forces — Newton's First and Second Laws
In this lesson you'll learn to: draw and label force diagrams from the
centre of mass, work out the resultant force (with its direction), use
W = m × g, explain what happens when forces are balanced (first law) and
unbalanced (second law), and use F = m × a.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. Drag the driving force and the resistive force on the car. When they match, what happens to its speed? When they don't? Then push a puck on ice along its motion and at right angles to it, and finish by sorting eight situations into the first or second law.
balanced…
Change the forces
Every arrow starts at the centre-of-mass dot. The length of an arrow shows the size of the force. Weight and the normal contact force always cancel here — it is the horizontal forces that decide whether the car speeds up, slows down or keeps going at a steady speed.
balanced…
Push the puck with a 1.0 N force — choose its direction
A dot is left every half a second.
Velocity is speed in a direction. A force along the motion changes the speed. A force at right angles changes the direction but not the speed — like a car on a bend at a steady 12 m/s. Either way the velocity changes, so it is accelerating: Newton's second law.
all three right first time 0 / 0
1 · Are the forces balanced?
2 · What happens to its velocity?
3 · Which law?
Physics With Kate · physicswithkate.com — Edexcel International GCSE Physics
▲ Try the exam-style button, then do the sums yourself before you read the panel: resultant first, then a = F ÷ m.
Resultant force zero? Nothing changes. Not zero? The velocity changes.
That one idea is Newton's first and second laws. The notes give you the named forces, how to draw them, how to find the resultant, and the two equations: W = mg and F = ma.
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Revision notes
What forces do
A force is a push or a pull. Forces act between two objects, and they can change an object's speed, its direction or its shape.
The forces you need to name
| Force | What it is | Direction |
|---|---|---|
| weight (force of gravity) | the pull of the Earth on the mass | down, towards the centre of the Earth |
| normal contact force | a surface pushing back on an object resting on it | at right angles to the surface |
| friction | two surfaces rubbing | opposes the motion |
| air resistance / drag | friction from air or water | opposes the motion |
| thrust / driving force | from an engine, a rocket or your legs | usually forwards |
| upthrust | a liquid or gas pushing up on an object in it | up |
| tension | the pull in a rope or string | along the rope |
| electrostatic and magnetic | forces between charges or magnets — they act at a distance | attract or repel |
⭐ Friction always opposes motion
Friction, air resistance and drag always act in the opposite direction to the way the object is moving (or trying to move). A car parked on a level road has no friction on it at all — there is nothing for it to oppose.
Drawing a force diagram
- Put a dot at the object's centre of mass — its middle.
- Draw every force as an arrow starting from that dot, pointing the way the force acts.
- Make the length of each arrow match the size of the force — bigger force, longer arrow.
- Label every arrow with its name and size.
Resultant force
The resultant force is the single force that has the same effect as all the forces together. For forces along a line:
- forces in the same direction — add them;
- forces in opposite directions — subtract the smaller from the bigger;
- always give the direction: "400 N forwards", "500 N downwards".
If the forces cancel, the resultant is 0 N and the forces are balanced. If not, they are unbalanced.
Weight
weight = mass × gravitational field strength — W = m × g. W in newtons (N), m in kilograms (kg), g in newtons per kilogram (N/kg). On Earth, use g = 10 N/kg. A 60 kg student has a weight of 60 × 10 = 600 N, acting downwards.
Newton's first law — balanced forces
⭐ First law
If the resultant force on an object is zero, the forces are balanced. The object stays stationary, or keeps moving at a constant velocity — the same speed in a straight line.
Balanced does not mean stopped. A car cruising at a steady 20 m/s and a skydiver at terminal velocity both have balanced forces on them.
Newton's second law — unbalanced forces
⭐ Second law
If the resultant force is not zero, the forces are unbalanced and the object accelerates: its velocity changes — its speed, its direction, or both. The bigger the resultant force, the bigger the acceleration.
resultant force = mass × acceleration — F = m × a. F in N, m in kg, a in m/s².
⚠ Trap 1 — a car on a bend at a steady speed is accelerating
Its speed doesn't change, but its direction does — so its velocity changes. That needs a resultant force, towards the centre of the bend. It is the second law, not the first.
⚠ Trap 2 — use the RESULTANT force in F = ma
Not the driving force. Work out the resultant first (driving − resistive), then divide by the mass. And convert kN to N: 5.4 kN = 5400 N.
⚠ Trap 3 — weight is a force, in newtons
Mass is in kg and never changes. Weight is in N and depends on g. "Weight = 80 kg" loses the mark.
How to write it in the exam
Explaining any motion — three links in the chain
The forces are balanced / unbalanced, so the resultant force is
zero / [size + direction], so the object
moves at constant velocity (or stays still) / accelerates.
compare the forces state the resultant say what the motion does
A boat has a thrust of 900 N and drag of 500 N. Calculate the resultant force.
Opposite directions, so subtract: 900 − 500 = 400 N forwards — the direction is part of the answer.
A car of mass 1200 kg has a driving force of 4.0 kN and a resistive force of 1.6 kN. Calculate its acceleration.
Convert: 4.0 kN = 4000 N, 1.6 kN = 1600 N. Resultant = 4000 − 1600 = 2400 N. a = F ÷ m = 2400 ÷ 1200 = 2.0 m/s²
A skydiver of mass 70 kg is falling at terminal velocity. What is the air resistance on her? Explain.
W = m × g = 70 × 10 = 700 N. At terminal velocity she moves at a constant velocity, so the forces are balanced and the air resistance = 700 N upwards.
Explain why a car going round a roundabout at a constant 10 m/s is accelerating.
Its direction is changing, so its velocity is changing. A change in velocity is an acceleration, caused by a resultant force towards the centre.
✅ Check your answer
Resultant force with a direction · kN converted to N · the resultant (not the driving force) in F = ma · weight in N, using g = 10 N/kg · units on every answer.
📘 Now do it in your workbook
Page 7: label six force diagrams, tick balanced or unbalanced, and calculate the weight and the resultant. Page 8: write the two laws, decide which law is acting, and do the F = ma exam question.
Edexcel iGCSE Forces and Motion Workbook · pages 7–8✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for these spec points.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the notes and the free workbook pages above cover every mark.
Now put it into practice
Pages 7 and 8 of the Forces and Motion workbook are free: six force diagrams to label and calculate, the two laws, and an F = ma exam question. The full book covers the whole topic with worked answers.
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